Test multiple configuration sets

The main goal of these changes is to allow running tests with various versions of ejabberd.cfg file.
You can define what ejabberd.cfg overlays are to be set for every configuration in test.config file.
Then the test specification will be run against every of given configurations.
If there is no "ejabberd_configs" entry in the configuration file, the test will be run only once with the default configuration as usual.

To make the testing process faster redundant backend tests in some modules (e.g. tests for mod_roster and mod_roster_odbc in one suite) have been removed.

Additionally, race conditions in muc_SUITE discovered during development have been removed.