Only set NovaWorkers in the non-default case

Only set NovaWorkers in the non-default case

This patch updates the t-h-t templates for
nova services so that we only set the value of workers in
the non-default case. TripleO has always defaulted the
workers count to 0 and there was recently a regression in
nova where they treat the default of 0 as invalid (a bug
that may get fixed in nova but we don't want to wait on it)

This patch avoids the issue by allowing the default value
to be unset if the TripleO default of 0 is configured.

Change-Id: I175977b88129d87caeb32332d47eb14816a6d5d4
Closes-bug: #1631133