Installer auto-detects distro, portable system init

The theme of this commit is to make the install process more
resiliant to differences in distros by testing for specific
features or versions of executables rather than making as many
assumptions just on the distro and release.

* Distro and Release detection (install, Vagrantfile)
  - Added detection logic for distro and release based on
    /etc/os-release which is available on all potential supported
    distros. The --distribution and --release options can be used
    to override detection.
  - Added distro and release checks to ensure values specified are
    for a supported release.
  - Changed some distro checks to use booleans, e.g.
    if $is_centos_7; then ...
  - Added a simple check so if --distribution is specified, it will
    ensure it's at least sane, e.g. centos on debian or debian on
    centos will be caught.
  - In Vagrantfile, removed --distribution and --release options for
    all distro and release combinations.

* Portable Init System Detection and Management (install, centos.sh)
  - Added detection logic for init system type - systemd, Upstart or
    System V in the function systemInitDetect().
  - Added portable init system install in the function
    systemInitInstall() which depends on systemInitDetect(). After
    installing files, enables and starts service.
  - Added portable init system commands in the function
    systemInitCommand() for start, stop, reload, restart and status.
  - Python services don't support systemd but unconditionally install
    scripts for both Upstart and System V. Disabled by passing
    --no-init-script to setup.py for each service.
  - When upgrading, remove all old system init files in /etc/init,
    /etc/init.d, /etc/default and /etc/systemd/system.
  - In centos.sh, removed install of airtime systemd service files
    as it's now handled by install.
  - Created an Upstart .conf for airtime-celery
  - In systemd file for airtime-celery, changed absolute path to
    /usr/local/bin/celery and for centos, the install symlinks
    celery to /usr/local/bin.

* External Dependencies Install (install)
  - For external dependencies, only attempt to install for Debian-
    based distros with apt-get. For centos, a warning is displayed.
  - For systems with apt-get, detect version and if 1.1 or greater,
    use new force options, otherwise use --force-yes option.

* Configuring Apache (install)
  - For Apache, check for centos and use httpd for binary and
    service, otherwise use apache2ctl and apache2.
  - Detect Apache version 2 or better in a more reliable manner.
  - Detect Apache root folder and conf file name by running
    apache2ctl -V or httpd -V as appropriate.
  - Various checks for centos as it doesn't support the Debian
    Apache utilities a2ensite/a2dissite, a2enmod/a2dismod, etc.

* Installing Airtime Services (install)
  - Detect Python version. If less than 2.7.9, install OpenSSL
    support.
  - Prevent installing init files by passing --no-init-script to
    setup.py for each service.
  - Use systemInitInstall to install, enable and start each service
    after setup.py has run.
  - Removed filtering for WEB_USER for files in
    /etc/init/airtime*.template as that is handled in the
    systemInitInstall() function.

* Configuring PHP in Apache (install)
  - Detect PHP conf folder by checking a list of locations rather
    than making assumptions based on the distro and release.

* Configuring PostgreSQL (install)
  - Detects if the airtime user has already been created. If not,
    then creates the user.

* Installing Locales (install)
  - Minor changes to check for centos and prevent from running.

python_apps/airtime-celery/install/upstart/airtime-celery.conf

@@ -0,0 +1,18 @@
+description "LibreTime Celery Service"
+author "help@libretime.org"
+
+start on runlevel [2345]
+stop on runlevel [!2345]
+
+respawn
+
+setuid celery
+setgid celery
+
+env LANG='en_US.UTF-8'
+env LC_ALL='en_US.UTF-8'
+env RMQ_CONFIG_FILE=/etc/airtime/airtime.conf
+
+chdir /srv/airtime
+
+exec celery worker -A airtime-celery.tasks:celery --time-limit=300 --concurrency=1 --config=celeryconfig -l INFO