Clean up disk space more tenaciously regardless of cluster status

[volodk85]

Description

A cluster might get stuck under the following conditions:

1. There are some unassigned shards and there are no available (with plenty of disk space) nodes to allocate. Cluster state is NOT green at this point.
2. In the same time, some nodes DO have disk space which is currently occupied by unassigned shard copies. This disk space could be used to allocate unassigned shards.
3. A node cannot start to reclaim disk space unless shard is green (and has no relocating copies)

[elasticsearchmachine]

Pinging @elastic/es-distributed (Team:Distributed)