Test multiple server configurations

Changes made enable tests to be run against multiple server configurations defined in test.config file without need to build the server again.
Overlays defined in test.config file are substituted in original configuration file and the server is restarted for each of those configurations.
Thus, multiple backend testing in some modules (like testing mod_roster and mod_roster_odbc in one suite) has been abandoned as it would cause tests redundancy and longer testing time as a consequence.
Additionally, race conditions in some test suites (especially muc_SUITE) have been removed.