ensure isolated test cases run with an isolated tenant

If some test cases run with the same tenant and in muiltprocess(using
'testr run --parallel'), race problem will raise. Because one test
case maybe delete the tenant in tearDownClass, but at the sametime
another test case run in another process have passed the setUpClass
stage, the AuthenticationFailure will arises. like this:
"AuthenticationFailure: Authentication with user
FlavorsTestJSON-user and password pass failed"

Part of blueprint speed-up-tempest

Change-Id: Ie840e02778165182461b2b0802db523b354a9b05
Merge "ensure isolated test cases run with an isolated tenant"