Refactor reschedule in exception handling of volume manager

Refactor reschedule in exception handling of volume manager

Previous exception handling has a pitfall that may potentially clear
the sys.exc_info() (by calling SQLalchmey to update db). This patch
refactors some part of exception handling in create_volume() to make
sure sys.exc_info() is retrieved so that we can log and reraise it;
also we make sure exception be reraised at the end of exception handling
no matter the request is rescheduled or not.

As a side effect, we fixed a bug in unittest which didn't provide
correct argument to db API but previously this exception has been wrongly
consumed by volume manager's exception handling (not reraise exception
when request is rescheduled).

fix bug: 1197648

Change-Id: Idce5d06f8be1fb6018012503ec7f844898a21b25