Allow building buildbot workers without a galera included

The Buildbot workers are used to generate galera packages and also by the server to perform MTR tests on galera.

For new platforms we have no galera build, which creates a problem building the worker.

Our github CI workflow needs a nogalera input so that there can be a build platform for the galera builder so it can create packages.

In the mean time the build worker won't have env WSREP_PROVIDER so it won't be automaticlly causing MTR failures for an invalid WSREP_PROVIDER.

After the galera package has been build, the boolean can be replaced for the next deployment.