subtests...rmi + dockertest.subtests + images fixes

*  A nakid ``except`` clause in the images module was hiding
   the cause of a failure in the rmi test.  The most likely
   exception to be thrown is a ``TypeError``, and so it was
   specified.

*  There's no practical reason why images w/o names but with
   valid Id's should be treated specially.  It's a perfectly
   acceptable intermediate state, but should not persist.
   Added a test in env checks to verify this.

*  Docker client version 1.1.1 reports the --run argument to
   the commit subcommand is deprecated.  Added version-check
   to bypass processing this parameter in rmi test.

*  Subtests.log_xn annoyingly complaining about multi-line
   messages that don't have any arguments.  Since this is
   a perfectly valid usage, the method needs to test the
   contents of args, and simply ignore it when empty.  If
   it's a problem, people will notice by the '%s' (or whatever)
   appearing in the output, no need to fail a test for this.

Signed-off-by: Chris Evich <cevich@redhat.com>