Do not install odl-restconf-nb-bierman02 by default

We are currently installing odl-restconf-nb-bierman02 as part of odl-restconf feature. As we are moving towards removal of this feature, remove it from odl-restconf, requiring users to explicitly to install it.

Also raise the severity of the warning issued since NETCONF-838 to highlight the fact the feature is considered obsolete and will be removed in the next major release.