Skip periodic cleanup if the node is not stale

* nodepool/nodepool.py(NodePool.periodicCleanup): Don't try to
delete nodes which have been in their current state for less than 15
minutes. Normal deletion waits up to 10 minutes before giving up, so
this avoids racing it in the periodic pass.

Change-Id: I0b96e792411dfcde5869cda0dbefc935ffdf5437