subtests...run_volumes: SELinux improvements

*  Clarified config. file comments
*  Updated documentation
*  Added additional checks for path lists
*  Made common method for changing contexts
   for all sub-subtsts.

Signed-off-by: Chris Evich <cevich@redhat.com>
subtests: Remove unused imports

Signed-off-by: Chris Evich <cevich@redhat.com>