GCP: small optimization (exploit the fact that 'None', empty strings and empty lists evaluate to 'False').

Core: small optimization (exploit the fact that 'None', empty strings and empty lists evaluate to 'False').

Core: small optimization (avoid collecting measures for sinks if the associated thread is not running, that is when no sink is active)

OpenStack: updated the authentication part used by Gnocchi and Ceilometer clients to fit recent updates in OpenStack

Chameleon: updated the authentication part used by Gnocchi client to fit recent updates in Chameleon Cloud

Removed use of 'common.utility.make_dict_proxy_wrapper' (previously used to make an OpenStack instance look like a dictionary) as it is no longer needed: an instance of OpenStack's openstack.compute.v2.server.Server class can now be used as dictionary as its parent class (i.e., the openstack.resource.Resource class) implements the mapping protocol.

Core: small optimization (avoid looking for measure sinks if the configuration parameter is an empty string)

GCP: added class to use for implementing synchronized operations (e.g., for waiting until the starting of an instance is done)

AWS: added class to use for implementing synchronized operations (e.g., for waiting until the starting of an instance is done)