Ensure repeated member deletion fails with 404

Fixes bug 1157427

If an image membership is deleted more than once, the second and
subsequent deletions would be expected to return a 404 Not Found
response code, as opposed to 204 No Content.

Previously, the can_show_deleted logic got in the way, such that
for an admin caller the previously deleted membership was considered
still extant.

We now override the show_deleted logic for deletion, as it makes no
sense to consider the existence of previously deleted database rows
when another deletion request is issued.

Note that there is no corresponding proposal to master as this logic
has been refactored and this no longer affects master.

Change-Id: If8200bbb74e46c814c9f8fa6b4c636e7b022ab6c
Check if creds are present and not None

Currently, KeystoneStrategy checks whether the required credentials are
present in the creds dict but it doesn't check whether they are valid or
not.

This patch checks whether the required creds are present and not None
otherwise a MissingCredentials exception will be raised.

Note: No need for checking parameters' values types since they'll be
instances of basestring once set otherwise they'll be None.

Fixes bug 1157765

Change-Id: I664a604c3cbf2fca60a88c4d887cd9a4b678c8a5
Merge "Ensure repeated member deletion fails with 404" into stable/folsom