Horizon English po files should be updated automatically

[Akihiro Motoki]

English PO files in Horizon repo are updated manually.
It is better to update them automatically by infra job.

The required tasks are:
- Fetch the latest horizon repo
- Run ./run_tests.sh --makemessages
- Propose a patch if there is a change of PO files

Note that we only need to update English PO files (English is the source language of translations)
and don't need to update other language PO files. It keeps the patch simple.

The infra job does not need to upload PO files to Transifex.
Transifex now watches the PO files in Horizon repo and fetch them if updated.

Regarding importing translations from Transifex, I18N need more discussion about when they should be imported.
Thus the translation import is out of scope of this bug.

[Akihiro Motoki (amotoki)]

horizon-upstream-translation-update.sh now works as post-merge job.

http://git.openstack.org/cgit/openstack-infra/config/tree/modules/jenkins/files/slave_scripts/upstream_translation_horizon.sh

https://jenkins.openstack.org/view/All/job/horizon-upstream-translation-update/