Improve existing client to support JSON.

UiPlugin class have all necessary methods
to manage UI plugins for given vCD and user.

Extension generator is class which inherits
generator abstract class and
give the ability to copy existing template from
the file system, of the user, and populate it
with the data, given by the user.

Validator classes gives the ability
to validate user or all other kind of
inputs.

Implement tests, which use pyvcloud test
framework, and verify all of them pass
successfully.

Signed-off-by: Nikola Vladimirov Iliev <nvladimirovi@vmware.com>