Searchable snapshots mounted in the 'hot' phase should be pinned to hot nodes

We added the ability to allow mounting an index from an ILM policy in the `hot` phase after rollover, however, these indices are automatically sent to the cold nodes. We need to change ILM such that we pass in `_tier_preference: data_hot` when they are mounted in the hot phase.

We'll also want to ensure they are migrated between phases appropriately.