Restore is:mergeable predicate in Gerrit

Gerrit 3.4.0 stopped generating the is:mergeable predicated by default,
but it seems to be rather helpful for some reviewers. The computational
load caused by this is O(N^2) where N depends on the number of changes
open against a branch and their respective size. Since most of the
changes we process are rather small and also we didn't see a significant
reduction in load when we moved to 3.4, this isn't expected to be an
issue in our installation.

[0] https://www.gerritcodereview.com/3.4.html
[1] https://gerrit-documentation.storage.googleapis.com/Documentation/3.4.0/config-gerrit.html#change.mergeabilityComputationBehavior

Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I9fce11b454255818e4a5817affed5b6e9c19f521

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

@@ -80,6 +80,8 @@
 	allowDrafts = false
 	disablePrivateChanges = true
 	move = false
+# Keep pre-3.4 behavior
+	mergeabilityComputationBehavior = API_REF_UPDATED_AND_CHANGE_REINDEX
 [receive]
 	maxObjectSizeLimit = 100 m
 	enableSignedPush = false