subtests...top: Clarify test image ps requirement

*  Simple default config and docs update
*  May be reverted in future if more general
   long-term solution found (e.g. automatic
   pre-test image building).

Signed-off-by: Chris Evich <cevich@redhat.com>
Doc. update: Remove redundant sections

*  Use of standard test-image options moved to
   defaults a long while ago, no need to list
   separately for tests unless they require them.
*  Same as above for ``remove_after_test`` option.
*  Cleaned out some sub-sections w/o any useful
   info.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #202 from cevich/topdocfix

Resolves #175