fix importing of optional modules in auth_token
fix importing of optional modules in auth_token

* keystone/middleware/auth_token.py: Catch the
correct exception so that the memcache and iso8601
modules can be optional as intended.
* tests/test_auth_token_middleware.py: Test
the ImportError path
* keystone/test.py: Add a new mixin class to
support disabling importing of a module.

Bug: 1003715
Change-Id: I87cc2f3bc79b17a52ea672bac7e0ebcf9e1fce57

varunarya10/keystonemiddleware