Refactoring of manager's create_volume flow

Refactoring of manager's create_volume flow

This commit resolves a TODO item (bulk metadata create), rephrases
some comments and makes use of new TaskFlow functionality to save
results of revert command to make decisions in c-vol manager instead
of doing an ugly workaround by injecting the information into exception
raised.

Partial-Implements: blueprint taskflow-refactoring
Depends-On: I82ebd0102aa5f50d98d9d6b48b13cd659d6dbcec
Change-Id: Ia4739c87ca83ae725e16256e3f2c596c5e6d935b