Improve isolation of order specs.

* Cleanup extra files created by individual specs.
* Change to the aruba tmp dir before running the
  command.  This is important so that it doesn't
  pick up options in our `.rspec` file.
Remove inconsistent #order and #randomize? config methods.

* Examples and groups can be ordered differently.
* Individual groups can be ordered differently.

As such, a global `order` reader method doesn't
make a lot of sense (although, assigning the global
default via `#order=` still does, I think).

Also:

* This exposed that the ordering force logic
  (e.g. to ensure CLI `--order rand` takes
  precedence over `RSpec.configure`) didn't
  work properly since it simply ensured `order`
  returned the forced option but did not ensure
  the global ordering was set properly.  I've
  fixed this.
* Don't clear the seed when setting the order to
  default. An individual example group may still
  be run in random order and will need the seed.
  There's really no reason to clear it.
* Base our decision of whether or not to print
  the seed at the end of the run on whether or
  not the random strategy ever got used, rather
  than whether or not `random` is the global
  strategy. If only one example group used the
  random strategy we would want it to print,
  but the old logic would not have done so.