Check for silent failure of tgtadm remove

In order to work around a failure to remove targets that
had a reconnect the force flag was added to the tgt delete cmd.
    (https://bugs.launchpad.net/cinder/+bug/1159948)

It turns out there's a bug in tgt where some versions will
sometimes silently fail when using the force flag.
    (https://bugs.launchpad.net/ubuntu/+source/tgt/+bug/1305343)

The problem is that in the gates since we merged the force change
there's a very high number of cases where lvremove fails because
the device is still active.  It appears that this is a result of
the silent force failures in target.

This patch adds a simple check after the force target removal,
if it detects the target is still present it reverts back to the old
non-force method to catch the cases that don't actually need the
force option.

This is a work-around until the version of target in the distros
is updated and can be used reliably, but closes the gate issue
bug that was reported.

Change-Id: I9150669040815e4831bd570964d12676b83ecbc9
Close-Bug: #1304122
(cherry picked from commit f9519182f4a6e6573513d8dbc9438702fc7b8644)