Make is_neutron() thread-safe

The utils method is_neutron() was not thread safe and could have returned the
wrong value if two greenthreads called it at the same time.  Specifically, a
greenthread switch could occur when the import is being done.  At that point, it
looks like the check has already been completed, but the result has not yet been
recorded.

Closes-bug: #1274992
Change-Id: I5a6eedaeb4c0deee4b31509946ae5daedf98389d
Merge "Make is_neutron() thread-safe"