Filter LDAP dumb member when listing role assignments

When the LDAP assignment driver is used and the "use_dumb_member"
configuration option is enabled, the dumb member is listed when
listing the role assignments. This can be seen by running the live
LDAP tests, as the "test_list_role_assignments_unfiltered" test will
fail due to the additional role assignments for the dumb member.

This patch filters out the LDAP dumb member when listing role
assignments.

Change-Id: I585d54847778f02621b8201644ec9285495a63aa
Closes-Bug: 1280965
expires_at should be in a tuple not turned into one

Change-Id: I273ed2cd984f1e69cc595169a59647c76a8ca202
Merge "expires_at should be in a tuple not turned into one"
Merge "Filter LDAP dumb member when listing role assignments"