Allow running test groups independently

We need to be able to run only a few test groups, even if we do not have all
database backends installed and configured locally.

ejabberd test suite configures a specific host per backend. I changed ejabberd
to allow ignoring some hosts from config file on start, by providing the exact
list of hosts we want to start.

This is done by setting an ejabberd app Erlang environment variable 'hosts' and
passing the list of hosts we want to actually define.

When doing so, the backend specific hosts defined in ejabberd test configuration file
are simply ignored. As a result, we do not try to connect to unavailable backends.

I linked that part to CT run test by defining the hosts list based on environment variable
CT_BACKENDS. This variable is expected to be a comma separated list of available backends.

When Erlang Common Tests are run with that environment variable set, only the host matching
the name of the backend will be set, plus the default "localhost", common to many tests.

This can be combined with rebar ct groups list.

Example commands to run tests:
CT_BACKENDS=riak,mnesia rebar ct suites=ejabberd
CT_BACKENDS=mnesia rebar ct suites=ejabberd groups=mnesia

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