Fix AutoScalingSnitch's use of usableSpace metrics to account for solr.data.home and dataDir in solrconfig.xml

SOLR-6671 and related issues SOLR-11036 and SOLR-11037 made me realize that computing free disk available in order to place new replicas on nodes is not as trivial as currently implemented.

Today we just use the metrics/solr.node/CONTAINER.fs.usableSpace metric which returns the usableSpace of the coreRootDirectory. But it has the following problems:

1. The solrconfig.xml of the collection's configset may specify a different path than coreRootDirectory
2. If solr.data.home is specified as a system property on the node then we must use that directory's usableSpace
3. For both of the above, we must also use the coreRootDirectory's usableSpace for correctness otherwise the node will flat out refuse to create a core because it cannot create a core.properties on disk.