[JBIDE-15115] implemented IOpenShiftResource#getMessages()

Implemented IOpenShiftResource#getMessages() and a Messages class in
order to hide internal Multimap structure required with new way of
reporting creation log/messages.
Also turned Severity into a class so that it gets extensible and allows
us to report unknown severties
Merge pull request #61 from adietish/JBIDE-15115

Merged by openshift-bot