Fix variables on "PXC/Galera Cluster Summary" dashboard

STR:

1. Run PMM2 server
2. Add PXC cluster to monitoring (use --addclient=pxc,2 --with-proxysql option fro was-staging-start job)
3. Open PXC/Galera Cluster Summary dashboard

Given result: There is no any cluster in Cluster drop-down list

Expected result: There is a cluster available in the Cluster list.

Original report: Cluster isn't filled up with data due to some formula issues.
It's required to restore this functionality

Here is the result after a fix