Skip test that rely on creds by role
Skip test that rely on creds by role

The legacy credential providers raises an exception when
is_role_available is invoked. This causes tests that rely on
credentials by role to fail rather than skip when that
credential provider is used.

The wrong behaviour was introduced in change
I351f86f0570070105bbf24f2d904e917feea6e57, and since than it's
causing periodic and experimental jobs using the legacy
credential provider to fail. Fixing that.

Change-Id: I0ca0b96fc618ffe60851984a9c6d46b0507878d4

cisco-openstack/tempest,bigswitch/tempest,openstack/tempest,sebrandon1/tempest,zsoltdudas/lis-tempest,LIS/lis-tempest,zsoltdudas/lis-tempest,bigswitch/tempest,Juniper/tempest,masayukig/tempest,vedujoshi/tempest,masayukig/tempest,vedujoshi/tempest,openstack/tempest,LIS/lis-tempest,sebrandon1/tempest,Juniper/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,Tesora/tesora-tempest