Fix client usage in multibackend volume test

This commit fixes the client usage in the multibackend volume api
tests. There was no reason for the test to initialize new instances
of the clients directly. Firstly this wasn't tenant isolation safe
and secondly the clients are already instantiated with admin credentials
in the base admin class setUpClass(). This commit just switches the
tests to use those clients instead.

Change-Id: Ifc986b82dcfe872ff1b25babd78ab90e996d5c12
Rename images variable in TempestConfigPrivate

The images variable in TempestConfigPrivate is the wrong name since the
group in tempest.conf is actually 'image' so rename it to match the
config file along with any usage of the variable.

Change-Id: I3144734942151c0e807c53e329709abcb9438167
Fix test_create_list_delete_volume_transfer without tenant isolation

The test was making an assumption that there would be exactly one volume
transfer listed which is not the case when running without tenant
isolation. Note the test before it asserts the listed transfers are
greater than or equal to one so it works without tenant isolation.

The reason this doesn't work in the single-threaded case is probably
because another test is not cleaning up properly so there is a leak
somewhere, therefore this is a partial fix until the leak is resolved.

Partial-Bug: #1277788

Change-Id: I68b2df2d7792da7c3a9ce001af41cd199ccdbc13
Merge "Fix client usage in multibackend volume test"
Merge "Rename images variable in TempestConfigPrivate"
Merge "Fix test_create_list_delete_volume_transfer without tenant isolation"