Move is_image_sharable to registry api

When using the membership kwarg in is_image_sharable with the sqlalchemy driver,
an AttributeError may be raised since the 'member' variable is not always
defined. This function was copied throughout the db drivers and untested.

This patch adds tests and moves the function to the v1 registry api, where it
was the only place being used. Tests for this function in test_context were
removed as it does not make sense to test this function there and allowed for
the removal of the membership kwarg.

Fixes bug 1247481

Change-Id: Ia77bc0a144831ddcf7e180afdb0842754ff3fcf2