Ensure rbd connect exception is properly caught

If the rbd driver fails to connect to Ceph the exception
was not being properly caught resulting in the volume
remaining in the 'creating' state until the corresponding
task eventually times out (on top of the time it took
for the connect to fail).

Also added config option for rados connect timeout.

Conflicts:
    cinder/volume/drivers/rbd.py
    etc/cinder/cinder.conf.sample

DocImpact: new config option 'rados_connect_timout'
(cherry picked from commit 02e4afbd8108ca26af09ccaebb8b1c8b72ea3f3f)
Change-Id: I5e6eaaaf6bed3e139ff476ecf9510ebe214a83f9