Control the number of workers for nova-conductor service

Closes-bug: #1286891

Change-Id: If2dd01b2267706b57e8b6b386b08bc9d9654cd90
Deprecate conductor_workers in nova::api

Since we now have a dedicated nova::conductor class, the
conductor_workers parameter in nova::api isn't at his right place.
This commit deprecate the conductor_workers parameter in profit of
workers param in nova::conductor ( see https://review.openstack.org/106656/ )

Change-Id: I767b5c143f12a0f518828287b33aedbfd2201e72

Conflicts:
	manifests/api.pp
	spec/classes/nova_api_spec.rb
Ensure conductor_workers stay at previous value

During the refactoring of the conductor_workers parameter from nova::api
to nova::conductor, the conductor_workers default changed from
$::processorcount to the parameter being absent.

While this is consistent with the nova default behaviour, it changes the
value for people who currently have a different default set. The solution is
to do nothing, in case no value is given.

For reference see:
https://review.openstack.org/#/c/106656
https://review.openstack.org/#/c/108941
http://docs.openstack.org/trunk/config-reference/content/list-of-compute-config-options.html

Change-Id: I908fc4953522f0b1db8f43882a44407aba9fa9f9