Remove devstack-check label and increase centos/py3k values

Both of these measures are intended to make allocation more fair in
the currently constrained environment.  The check jobs have an advantage
in being able to run on two node types, and nodepool may be favoring
them due to that.  Remove the check label for now.

Set the min-ready for centos and py3k to values that are closer (50%)
of the precise nodes.  Reducing the spread should make the allocator
distribute nodes more closely according to current demand.

These are untested suppositions, but I don't think they will hurt.
They may help.

Change-Id: Id228c7e0f3422a7432af84b3f3bd1817b3b9b139