Characterize image_member_find

Add tests to characterize the current (incorrect) behavior of
image_member_find in the database drivers.

Change-Id: I0fc3bb05dbc1376d3bd8e002306b402e960780cf
Fix calls to image_member_find in tests

A few tests of image members in v2 were passing invalid arguments to
image_member_find. This problem showed up in a later patch where
image_member_find actually tries to make use of the context it was
passed.

Change-Id: I0611db9bf83ebf262e9d2a0ef33c805d23658f2a
Merge remote-tracking branch 'm_src_repo/webob-issue' into build-temp