manager: refactor of joinParticipant method

- decrease cyclomatic complexity of joinParticipant by
factoring conference binding code and removing
unneeded createConference method.

- move the ConferenceCreated signal at the right place
in the flow (i.e. last in sequence).

Based on a proposition from zinspierre <zins.pierre@gmail.com>
and al. in part of the course LOG8430 (Polytechnique Montreal).

Thanks to them.

Change-Id: I55203cc400496cd679c4f157d765aee0dec9c4f8
Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>