Fix pep8 failures in manila/{db,volume}

Fix pep8 failures, especially:
H402  one line docstring needs punctuation.
E127 continuation line over-indented for visual indent
E128 continuation line under-indented for visual indent
F401 'xxx' imported but unused

Partial-Bug: #1333290
Change-Id: Ia285d30574c3ad51b839823ea934e274f26ce6a3
Merge "Fix pep8 failures in manila/{db,volume}"