Add TransportKey as a resource

This is the first step in implementing key wrapping from the client.
Transport Keys will be public keys/ certificates which will be
fetched for plugins when storing/retrieving secrets.

Change-Id: I2a71b95f1a3f28bf6671de3ea9dc858218ff4b0d
Implements: blueprint add-wrapping-key-to-barbican-server
Merge "Add TransportKey as a resource"