job_add_git_tag is added.

job_add_git_tag is added.

Changes:
- job_add_git_tag.py is added
- job_submit.py is modified.
  - This is because pushing a tag is done by admin
  - Using Forge Committer Identity in gerrit,
    tagger information will be forged.
  - Original job_submit.py uses 'gerrit_account_name'
    that is generated by gerrit, and it is not forged.
  - To get forged identity, we have to use
    git for-each-ref --format=%(tagger).

Change-Id: I6ff87154a3aa31493c4d329e2726ba32f7ba3303
Signed-off-by: Junghyun Kim <jh0822.kim@samsung.com>