Introduce role-specific nova-server-metadata

Introduce role-specific nova-server-metadata

We could already pass metadata to the nova server instances (on
creation) via the ServerMetadata parameter, however, there was no
way of doing this per-role. This introduces that by adding a
{{role}}ServerMetadata parameter for each role. This parameter gets
merged with the ServerMetadata parameter and allows this
functionality.

Note that both default to {}, and so does the result of merging those
parameters with their default values. So nothing changes for the
default settings.

Change-Id: I334edcc51ce7ee82fc13b6cf4c0d74ccb7db099c