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.
Use correct path for rabbitmq-plugins by using Config::CONFIG["libdir"]

This will make it work on more distributions (since the path might be a
bit different, depending on the distribution: /usr/lib vs /usr/lib64,
for instance).
Merge pull request #7 from aspiers/server-role-validation

ensure that the rabbitmq-server role is assigned in rabbitmq proposals
Merge pull request #8 from vuntz/rabbitmq-plugins-path

Use correct path for rabbitmq-plugins on SUSE
Use RbConfig::CONFIG instead of Config::CONFIG

The latter just doesn't work.
Merge pull request #11 from vuntz/fix-libdir

Use RbConfig::CONFIG instead of Config::CONFIG