Docker:  Allow skip subtests from defaults.ini file

*  Add skip_subtests to control to scan
   skipped subtests from defaults.ini.

Signed-off-by: Ouyang Guohua <gouyang@redhat.com>


it seems can disable single case by add enable = false to its config file, but it's not conflict with this feature.
Here I just want to skip some subtests by some reason, such as the subtest consume much time, or it's not fulfilled yet, or even it's has some impact on other subtests due to not well cleanup.