powervm: Use FixedIntervalLoopingCall for polling LPAR status

This patch moves the code in stop_lpar that polls the LPAR status for
'Not Activated' to it's own helper method that uses
FixedIntervalLoopingCall with an eventlet timeout.

Also moves the hard-coded timeout to the constants file.

Builds on top of change Iad64dda79f46d18c7aa5bee12ed41e7a01853d34 to
verify the new looping call and timeout code works with the
test_stop_lpar_timeout test case in that patch.

Change-Id: I59586d3cb242d40ee5e09e48274de11cb5638717
Merge "powervm: Use FixedIntervalLoopingCall for polling LPAR status"