ejabberd: wait until ejabberd is up and running

ejabberdctl start returns immediately even if ejabberd is not ready
yet. Add a call to ejabberdctl started just after to wait until the
status says ejabberd is up and running.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>