Apex,Functest: Updates env detection

If Apex installer VM doesn't exist (such as in snapshot deployments) and
the credentials file is already provided, then can continue functest.
This patch adds that functionality and removes detection for the
"instack" installer VM which is deprecated.

Change-Id: I4267aebbcf0582529f38d451a7e8ba30f71c132a
Signed-off-by: Tim Rozet <trozet@redhat.com>