client: add the support for custom entity parsers

Some API (e.g. glance image show and download) require custom entity
parsers.

In this patch:
* OpenStackClientConnector.request() (renamed from execute) returns now
  OpenStackResponse objects
* the OpenStackResponse interface has now a getEntity method to parse
  and return the entity
* the connectors and responses implementations (resteasy and jersey2)
  have been adapted to the new interfaces
* a new RESTEasyResponse class has been added
* OpenStackClient now exposes a request method which returns response
  objects
* the old OpenStackClient execute method is now a wrapper to the request
  and getEntity methods
* the GlanceListImages examples has been updated to test show image

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>