Set default API_WORKERS

Set the API_WORKERS default to control memory usage. Maximum is nproc / 2 and
minimum is 2.

* Also updates https://review.openstack.org/#/c/117517/ to remove the
  conditional test as API_WORKERS should always be set.
* Update https://review.openstack.org/#/c/109058/ for ceilometer to use API_WORKERS

The following reviews can move forward either as-is or with minor tweaks:
* Keystone: https://review.openstack.org/#/c/121384/ - remove the if check
* Swift: https://review.openstack.org/#/c/121456/ - unabandon, the default
  to 1 is fine, or remove it to match the others.
* Trove: https://review.openstack.org/#/c/121438/ - remove the if check

https://etherpad.openstack.org/p/devstack-workers has the details

Change-Id: Id28d72ebf01c88b7df301edf7d1dd7ec23fcd0d6
Merge "Set default API_WORKERS"