Patch sent by Vitaly Kuznetsov to handle issue he found.
Merge pull request #12 from vittyvk/master

Fix test_Populate to comply 'nightly' katello build
tests/test_Populate.py: assume os=RHEL if not specified
system.py: add refresh_subscriptions and systems_by_org api calls
test_HealAll introduced (refreshing all systems subscriptions in org)