Fixes RequestContext initialization failure.

RequestContext initialization failed in nova because of the following
error:
"TypeError: 'in <string>' requires string as left operand, not NoneType"

It must traverses in tuple not in string when find the "volume" service_catalog.

Change-Id: Ie0de50fcafbeabf4698696add610d4e6804f45eb
Closes-Bug: #1248424
Merge "Fixes RequestContext initialization failure."