Clean up our gerrit config

This removes the old config to choose the old change screen by default
as everything is polygerrit now.

We remove the pre plugin melody config as melody now ships as a plugin
and has separate configuration.

We remove old theming information as that is supplied via external files
now.

We remove anonymous git download config because we don't set
gerrit.canonicalGitUrl which is required for this to work. We don't set
that because we don't have a git:// server anymore.

Bump the lucene thread count from 4 to 8 as we have more cores on the
system we run on.

Finally add some comments to help make sense of config that is left in
place.

Change-Id: Ie0b48e544191839067e66647d2ea32f74ce19ed3

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

@@ -4,7 +4,6 @@
 [gerrit]
 	basePath = git
 	canonicalWebUrl = https://{{ gerrit_vhost_name }}/
-	changeScreen = OLD_UI
 	reportBugText = Get Help
 	reportBugUrl = "https://docs.openstack.org/infra/system-config/project.html#contributing"
 	gitHttpUrl = https://{{ gerrit_vhost_name }}/
@@ -72,6 +71,7 @@
     memoryLimit = 4096
 [cache "web_sessions"]
 	maxAge = 7days
+# Set to @openstack.org because @opendev.org lacks a valid MX
 [user]
 	email = review@openstack.org
 [change]
@@ -129,18 +129,6 @@
 	match = "\\#?(\\d+)"
 	footer = task:
 	system = Storyboard
-[theme]
-	backgroundColor = ffffff
-	topMenuColor = ffffff
-	textColor = 264d69
-	trimColor = eef3f5
-	selectionColor = d1e6ea
-	changeTableOutdatedColor = f5cccc
-	tableOddRowColor = ffffff
-	tableEvenRowColor = f5f5ff
-[melody]
-	monitoring = true
-	session = true
 [plugin "javamelody"]
         allowTopMenu = false
 # Gerrit upstream hardcodes a .git extension for cgit.
@@ -152,7 +140,7 @@
 	revision = "?p=${project}.git;a=commitdiff;h=${commit}"
 [index]
 	type = LUCENE
-	threads = 4
+	threads = 8
 [download]
         command = checkout
         command = cherry_pick
@@ -160,11 +148,11 @@
         command = format_patch
         scheme = ssh
         scheme = anon_http
-        scheme = anon_git
         archive = tar
         archive = tbz2
         archive = tgz
         archive = txz
+# Plugin commit-message-length-validator config
 [commitmessage]
         maxLineLength = 72
 [groups]