ensure that the rabbitmq-server role is assigned in rabbitmq proposals

In the future we will hopefully support HA rabbitmq, where there could
be multiple servers.

Currently there's only one possible validation error, but we follow
the pattern used by other barclamps for building an errors array and
joining it, to allow easy addition of other errors in the future.
Merge pull request #7 from aspiers/server-role-validation

ensure that the rabbitmq-server role is assigned in rabbitmq proposals