Handle missing config options for tests gracefully

Running a subset of testcases (i.e. with "run_tests.sh -d
manila.tests.test_share_generic") leads to NoSuchOptError. Just ignore
tthis error and assume that the option is not needed because it's not
imported.

Co-Authored-By: Valeriy Ponomaryov <vponomaryov@mirantis.com>
Change-Id: Ib739452b139d9c2d7302c4a06336787890c50bb1
Merge "Handle missing config options for tests gracefully"