This allows `path.shared_data` to be added to the security manager while
still allowing a custom `data_path` for indices using shadow replicas.
For example, configuring `path.shared_data: /tmp/foo`, then created an
index with:
```
POST /myindex
{
"index": {
"number_of_shards": 1,
"number_of_replicas": 1,
"data_path": "/tmp/foo/bar/baz",
"shadow_replicas": true
}
}
```
The index will then reside in `/tmp/foo/bar/baz`.
`path.shared_data` defaults to `null` if not specified.
Resolves#12714
Relates to #11065
ff5ad39c7a