Increase the number of Gerrit threads for http requests

We've seen CI systems consume all of our threads which causes the web UI
to become non responsive. To address this increase the number of httpd
threads from 100 to 150. Note that we do not modify sshd.threads beacuse
sshd.threads determines the max number of git requests across both ssh
and http.

In theory what this means is that httpd has an additional 50 threads to
process non git requests (for example web UI requests) which will
hopefully keep that responsive even if git requests are max'd out.

It is possible that we also need to increase the sshd.threads value to
handle those git requests, but we will start by modifying one config
value at a time. If we do bump sshd.threads we should increase
httpd.maxThreads to give it that additional headroom.

Finally, I believe this is likely to be safe as we doubled the size of
our Gerrit server when we moved it to vexxhost. The old server was
pretty well maxed out though so increase these values on the new server
slowly and monitor the results.

Details on the configuration can be found at:

  https://gerrit-review.googlesource.com/Documentation/config-gerrit.html#httpd

Change-Id: I57a1e248c3c01597bb29c7afc304688e834a64cc

playbooks/roles/gerrit/templates/gerrit.config.j2

@@ -53,7 +53,7 @@
 [httpd]
 	listenUrl = proxy-https://*:8081/
 	minThreads = 20
-	maxThreads = 100
+	maxThreads = 150
 	maxQueued = 200
 	requestLog = true
 [tracing]