Break up backupIsReady in SolrCloud status by backupRepository

[HoustonPutman]

There is also a much easier way to calculate this.

When we generate a Solr pod spec in the StatefulSet, that pod spec should "be ready" for all backupRepos currently configured. So if we set an annotation on the pod for each repo stating it's most recent version, then we should be able to easily get the annotations on each pod to see the backupRepos that are able to be used on each pod. Much better than scanning for various volumes, or whatever else that specific BackupRepository implementation might require.

[HoustonPutman]

Relates to #301 now that you can define multiple backup repositories.