Issue title: Disk-based shard allocation does not relocate shards unless path.data is absolute
 Issue body: If `path.data` is set to a relative path then `clusterInfo.getDataPath(shardRouting)` also returns a relative path. However, `getDiskUsage()` returns an absolute path. This mismatch effectively disables the disk-based shard allocator at this condition:

https://github.com/elastic/elasticsearch/blob/88c9ecb0b06c0c1951900e162687fdd8c66bd972/server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java#L278

 Comments: 
 Comment 0: Pinging @elastic/es-distributed
 Comment 1: I think that we should be normalizing all the paths in the cindy04@example.com. Adding the core/infra label too.
 Comment 2: Pinging @elastic/es-core-infra
{'source': 'GithubIssues', 'repo_name_with_owner': 'elastic/elasticsearch', 'issue_id': 476499033, 'repo_stars': 61711.0}