Add usage note in utils.Timer class
Add usage note in utils.Timer class

This class, when a timeout is defined, cannot be used in other
than the main thread. When a timeout is defined, an alarm signal is
set. Only the main thread is allowed to set a signal handler and the
signal handlers are always executed in this main thread [1].

[1] https://docs.python.org/3/library/signal.html#signals-and-threads

Change-Id: I9ffdde96873fa3a557e4a4ddeeb9abdf89d3045a
Related-Bug: #1832925

mahak/neutron,mahak/neutron,openstack/neutron,openstack/neutron,openstack/neutron,mahak/neutron