Use cinder.utils.execute directly

Use cinder.utils.execute directly

When we were in the process of working on brick libs
we did quite a bit of funky stuff with setting a member
variable for each class to point to an executor that was
passed in during init.

There's no longer any reason to do this with the target drivers,
so we should simplify our lives a bit and go back to using the
good old cinder.utils wrapper.

It's also believed that the use of the member variable is
susceptible to some concurrency issue that was causing the
wrong cmd string to be executed.  We'll mark this as closing
that bug and reopen if we still see the signature in Kibana.

Change-Id: I7a5648d496d15eec5d7ac3643411198226f1ffdf
Closes-Bug: #1398078