Property methods for resource metadata

For OS::Nova::Server which will build its metadata dynamically
based on currently available SoftwareDeployment resources.
This change makes it possible to:
* read and assign resource.metadata as before
* subclassses to override resource.metadata for building dynamically
* allowing access to the simple attribute based resource._metadata
  when necessary

partial blueprint: hot-software-config
Change-Id: Ifd7aa15dd58c96e6b9efafe6b4201a964e140d8d
Merge "Property methods for resource metadata"