Create the DbAdapterFactory to instanciate db adapters, add resources.ini to configure resources, change
the authentication Manager to fall back to backends with lower priority in case
of errors, update the current UserBackends to the changed environment. Also
adjust the documentation and existing unit tests.
resolves#4503
Set the default database and user to "icingaweb" in the authentication.ini. Add
instructions about how to set up the databases to authentication.ini.
refs #3769
Add functionality to check if a certain database type like psql or mysql is
available and skip the tests accordingly.
Add documentation for backend authentication.
refs #3769
