Rework Passenger support for Debian and update docs

This will undo the changes introduced in PR #711 because they are not
useful or necessary for Debian and Ubuntu, and did not work correctly
anyway. Fixes MODULES-1012.

Set correct default values for PassengerRoot, PassengerRuby and
PassengerDefaultRuby depending on the Debian or Ubuntu release. For
Debian 7 and Ubuntu 12.04 PassengerRoot is '/usr' while for Ubuntu 14.04
it is '/usr/lib/ruby/vendor_ruby/phusion_passenger/locations.ini'.
PassengerRuby is '/usr/bin/ruby' for Debian 7/Ubuntu 12.04 while
PassengerDefaultRuby is unset, because this directive does not exist for
the Passenger verions packaged for these operating systems. For Ubuntu
14.04 PassengerRuby is unset and PassengerDefaultRuby is
'/usr/bin/ruby' instead.

Includes partially rewritten and updated Passenger documentation in
README.passenger.md, explaining the way things are handled for
Debian/Ubuntu and CentOS, where 'passenger.conf' is left untouched and
'passenger_extra.conf' contains the Puppet-managed Passenger
configuration. Also explains the user's responsibility to set
PassengerRoot correctly when using third-party or custom Passenger
packages, as the defaults only work for the Passenger versions and
packages shipped with the operating system.

Since Debian and Ubuntu ship with libapache2-mod-passenger packages that
will not change the packaged Passenger version over time (unlike EPEL
might) there is no need to tip-toe around the 'passenger.conf' file
installed by these packages.

Also contains updated and cleaned up Passenger spec and acceptance
tests.