Auto degrade sessions to operator when admin is forbidden

In certain circumstances, the account desired for use with pyghmi will
be merely operator.  Accomodate this without configuration required by
going for admin and then falling back to operator by default.  For now
try to go no lower than operator.  It would be conceivable to go down
to user, but most everything one would want a service processor for
at that point isn't feasible anyway.

Change-Id: I775aee5db0ca5b8f18bebc2210d342e3d629016d