heat tests : convert functional tests to folsom glanceclient

Convert functional tests to work with folsom glanceclient, also
remove the conditional essex/folsom module import since this will
no longer work on essex (this was bogus anyway since this code can
never have worked on folsom)

Note : this breaks compatibility with essex for the functional tests, so
any platform running them must be running >= Folsom from now on.

(The essex/folsom glance client interfaces are too different to make it worth
supporting both for the functional tests, ref heat-jeos..)

Change-Id: I4463bfab7f8acf93b2eafa4bb702bf86d8c78652
Signed-off-by: Steven Hardy <shardy@redhat.com>