Skip test_list_servers_by_admin_with_all_tenants test

This patch add skip decorator for test_list_servers_by_admin_with_all_tenants.
After use 'all_projects' instead of 'all_tenants', will fix this test.

Related Patch: https://review.openstack.org/#/c/58292/

Change-Id: Ibb71c3a966f6c09b93e821781d4266d56d65950b
Related-Bug: #1265416
Merge "Skip test_list_servers_by_admin_with_all_tenants test"