Fix volume region in the UI

Support calling RunCollection.get_plan() without repo (dstackai#1741)

Support AWS placement groups for cluster fleets (dstackai#1725)

* Support aws placement groups for cluster fleets

* Test placement group processing

* Adapt placement groups to work with dstack sky

* Update docs

* Fix tests

Fix SSHAttach.reuse_ports_lock() when no grep matches (dstackai#1700)

Fixes: dstackai#1689