Update B2Qt RTA triggering to once a night

Use file http://qt-online-staging.intra.qt.io/enterprise/qtsdkrepository/all_os/boot2qt/640/b2qt_640/Updates.xml as the TRIGGER_URL in trigger jobs and schedule triggering to 6-7am.

Trigger job template needs modifications to the "spec" in

    <org.jenkinsci.plugins.urltrigger.URLTrigger plugin="urltrigger@0.45">
      <spec>H H(2-4) * * *</spec>

define variable which default value is the current "H H(2-4) * * *" which is used instead of hardcoded value, so it's possible to modify for the jobs.

This could be applied for 6.4 for now and see how it works, if proven to be better than the current apply also to other versions