Feature/link aggr capab getters

 Getters implementation in LinkAggregationCapability.

They make use of LinkAggregationAPIAdapter to translate from model objects to defined API ones.

Fixes issues:
http://jira.i2cat.net:8080/browse/OPENNAAS-1331
http://jira.i2cat.net:8080/browse/OPENNAAS-1380

To be merged after #889