Decouple resources and preconditions in secgroup scenario

Handle secondary tenant checks based on CONF file only instead of using clients

Skip only cross-tenant test instead of whole class if secondary tenant is not
available

Change-Id: I9863ff840b9c5b18fa46826aab3fc107454dcc24
Fixes grammatical errors in HACKING.rst

There are a few lines in HACKING.rst that were incorrect
grammatically. This fix changes those.

Change-Id: I8173dae3cf3a3e78a504b4edd4a7c1ff5b7fb9a4
Closes-Bug: #1389603
Stop resource leakage for negative tests

get_client_manager should only create a new isolated_creds class if
there isn't already a isolated_creads instance bound to the test.

Change-Id: I44fd05d3c1dc713ec7bd176af689b008fbb5d7eb
Fixes-bug: #1371706
Merge "Stop resource leakage for negative tests"
Merge "Fixes grammatical errors in HACKING.rst"
Merge "Decouple resources and preconditions in secgroup scenario"