examples: add the GlanceListImages example

This patch introduces a new example for the glance module. The example
authenticates to a known keystone instance to query the catalog and
retrieve the "image" (glance) services list.
Then it connects to the first glance instance and retrieves the list of
the available images.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
build: address all the build warnings

In order to better identify possible issues introduced by future
patches, all the current build warnings have been either fixed or
suppressed.

* the serialVersionUID member has been added for the serializable
  objects
* the imports have been organized (removing the unnecessary ones)
* get methods have been provided for the unused private memebrs
* unused code has been commented

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
api: remove the WebTarget dependency

In order to remove the dependency on the WebTarget class from the
api commands, the following changes has been introduced:

* the execute methods have been removed from all the Client classes (it
  is now unified in AbstractOpenStackClient)
* the Command interfaces have been removed (their use was internal only)
* all the Commands implement the newly introduced OpenStackCommand (the
  only interface now depending on WebTarget)

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
api: initial removal of the jax-rs dependencies

In order to remove the jax-rs dependencies (WebTarget, MediaType,
Entity, etc...) the new OpenStackTarget class has been introduced.
The OpenStackTarget class objective is to abstract the get, post,
put, delete requests so that it will be possible to use different
versions of the jax-rs specification.
In those cases where removing such dependencies was too complex
(e.g. uploading/downloading an image in glace, the swift module,
etc...) the OpenStackTarget.getWebTarget method has been used in
order to retrieve the WebTarget object and maintain the previous
implementation. In the future also these corner cases will be
abstracted and handled in the proper way.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>