Add hook to generate metadata from service profiles

Add hook to generate metadata from service profiles

This enables the deployer to dynamically add nova metadata to the
servers based on the output of service profiles that implement the
metadata_settings key in the role_data output for the profiles.

One can set an implementation via the OS::TripleO::ServerMetadataHook
resource, which currently is set as OS::Heat::None. So, because of
the default implementation, if left untouched it actually does
nothing.

Currently, besides the list, which is metadata_settings, this hook also
takes the name of the node that it's setting the metadata for.

This is useful for nova vendordata plugins that can parse said metadata.

Change-Id: I8a937f711f0b90156fbb6c4632760435ef846474