use nproc/2 workers for large ops job

Commit 1ce19ab76d67a89b04f907f1d292d013a3b699e0 dropped API_WORKERS from
nproc/2 to nproc/4 and also started using API_WORKERS for the number of
conductor workers, so in gate runs that dropped conductor workers from 8
to 2.  We're now seeing instance build timeouts in the large ops job.

This change goes back to nproc/2 for the large ops job (VIRT_DRIVER=='fake').

Closes-Bug: #1500615

Change-Id: Ie6ef855fce0a99c930d479b7459c15b69e8de499
diff --git a/stackrc b/stackrc
index e010b45..c7c6313 100644
--- a/stackrc
+++ b/stackrc
@@ -650,7 +650,12 @@
 # the memory used where there are a large number of CPUs present
 # (the default number of workers for many services is the number of CPUs)
 # Also sets the minimum number of workers to 2.
-API_WORKERS=${API_WORKERS:=$(( ($(nproc)/4)<2 ? 2 : ($(nproc)/4) ))}
+if [[ "$VIRT_DRIVER" = 'fake' ]]; then
+    # we need more workers for the large ops job
+    API_WORKERS=${API_WORKERS:=$(( ($(nproc)/2)<2 ? 2 : ($(nproc)/2) ))}
+else
+    API_WORKERS=${API_WORKERS:=$(( ($(nproc)/4)<2 ? 2 : ($(nproc)/4) ))}
+fi
 
 # Service startup timeout
 SERVICE_TIMEOUT=${SERVICE_TIMEOUT:-60}