Set workers=$API_WORKERS in glance-registry.conf

We use $API_WORKERS to throttle the number of workers
in other services but were not doing it for g-reg for
some reason, which by default will run ncpu workers
up to a limit of 8.

Change-Id: Idc81ce05546e6d625c10e2229256eafbe7c057a5
Closes-Bug: #1774781
1 file changed