management: Skip hourly crons during deploys.

This is most important for `send_zulip_update_announcements`, which
can race with the version run as a post-deploy hook.  However, all of
these crons can tolerate being slightly delayed, and there's little
benefit to them taking CPU or possibly hitting odd borderline race
conditions when the deploy is in progress.

For safety, we only trust the deploy lockfile if it was created
within the last hour -- deploys should not take more than an hour, and
failing to ever run hourly crons is much worse than perhaps running
them during a real very-long deploy.

puppet/zulip/templates/cron.template.erb

@@ -2,6 +2,7 @@ MAILTO=zulip
 PATH=/usr/local/bin:/usr/bin:/bin
 SHELL=/bin/bash
 USER=<%= @user %>
+RUNNING_UNDER_CRON=1
 <% if @proxy != '' -%>
 HTTP_proxy="<%= @proxy %>"
 HTTPS_proxy="<%= @proxy %>"