fix(instances): a clone takes a free name the form offers

A user who is not a superuser chooses the clone's name from the free
names the clone form lists, or gets the first one with automatic names,
but the server took any posted name. It now refuses a name outside the
list, and with automatic names it takes the first free one whatever is
posted. A superuser still names a clone freely.

The panel checks VM names on every compute, but a job held a name only
on its compute, so two clones from VMs on two hosts could take the same
free name at once; a name claim now holds the name everywhere. A clone
job whose name a VM found on a host took while it was queued stops
before it copies anything.

operations/claims.py

@@ -48,8 +48,9 @@ def volume_key(compute_id, pool, name):
     return f"volume:{compute_id}:{pool}/{name}"
 
 
-def name_key(compute_id, name):
-    return f"name:{compute_id}:{name}"
+def name_key(name):
+    """A VM name is the panel's on every compute (the name checks are global)."""
+    return f"name:{name}"
 
 
 def quota_key(user_id):