please add support for applying graphs and dashboards to clusters

[kellyjmk]

From the examples:
syncsreg_exceptions:
title: "syncsreg exceptions"
targets:
syncsreg.log.exception:
🔑 exceptions/s
:color: red
hosts: ["wp-sreg*"]

In our environment, we don't have hosts named "wp-sreg_", we have hosts named aaa1,bbb2,aaa2, which might all live in a cluster called "wp-sreg". Please add support for a clusters: key so in the above example I could use 'clusters: ["wp-sreg_"]' to determine what graphs and dashboards are applicable to what groups of hosts.

Thanks!