Add tempest test for heat resource OS::Nova::KeyPair

The template will create two OS::Nova::KeyPair and will test that:
   [*] The key with save_private_key flag set to true outputs both
       public and private keys.
   [*] The key with save_private_key flag set to false outputs
       only the public key.

Testing other cases will require to recreate the stack and
include a negative test case.

Change-Id: I751a85e0efd997a1238c53d8802ebdd8b551e7aa
Partial-Bug: #1271995
Merge "Add tempest test for heat resource OS::Nova::KeyPair"