Feature/remove offwd rule

This pull request introduces two features:
- removeOpenflowForwardingRule
- refresh model flows in create/remove/get OpenflowForwardingRule(s)

It does so updating OpenflowForwardingCapability by:
- Implement removeOpenflowForwardingRule public method
- Implement refreshModelFlows private method
- Call refreshModelFlows from create/remove/get OpenflowForwardingRule(s)
- Small refactor 

Fixes issues:
http://jira.i2cat.net:8080/browse/OPENNAAS-1116
http://jira.i2cat.net:8080/browse/OPENNAAS-1123
http://jira.i2cat.net:8080/browse/OPENNAAS-1130
http://jira.i2cat.net:8080/browse/OPENNAAS-1131