Supporting several connectors for the same OCCI extension simultaneously

[philippemerle]

Context:
One OCCI extension can be implemented by several connectors. For instance, the OCCI Infrastructure Extension is implemented by two connectors: one for PCA, another for VMware.

Problem:
One running Cloud Designer instance can not support several connectors for the same OCCI extension simultaneously.

Cause:
Each OCCI extension is mapped to one EMF package.
Each connector registers a factory for a given EMF package (i.e., OCCI extension).
In EMF, only one factory can be associated to an EMF package.
Then two connectors register their factory but only the last one is kept.