jenkins: Downgrade k8s plugin

Version 1.26.1 may be the latest, but it apparently has major breaking changes
that cause the jenkins operator to fail to schedule jobs. Unclear why, but
upstream operator suggests running 1.15.2. We'll run 1.15.10 to include some
security fixes and be "more up to date" rather than stale.

Change-Id: I06724c84197a5d7f5401ddb1499927d28a158879
1 file changed