Fix alt tenant id's issue in BaseV2MemberImageTest

BaseV2MemberImageTest gets alt_tenant_id in the wrong way when tenant
isolation is off. commit 9612e81d142e88782d4faf6a85e81967b5323908
introduced getting the tenant_id from credentials instance. This case
was missed. Fixing this involves completely removing the code that
caused lp#1320079.

Change-Id: Ic8585077b876f70353202f0f47184a18bca8083d
Closes-bug: #1320079
Merge "Fix alt tenant id's issue in BaseV2MemberImageTest"