update-jenkins-jobs could use outdated code when triggered by gerrit merge

[Igor Belikov]

update-jenkins-jobs will use outdated version of jenkins jobs when triggered by merge of patchset not rebased on current master. This can cause a regression when recent fixes to jenkins jobs will be reverted by merge of some patchset not rebased on current repo master.

[Fuel Devops McRobotson (fuel-devops-robot)]

Fix proposed to branch: master
Change author: Igor Belikov <email address hidden>
Review: https://review.fuel-infra.org/16232

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/16232
Submitter: Igor Belikov <email address hidden>
Branch: master

Commit: e69f00a79e1feb1c3fbd2b8204624609c6a00f44
Author: Igor Belikov <email address hidden>
Date: Tue Jan 19 09:46:48 2016

Force master checkout for update-jenkins-jobs

This patch forces checkout of current jenkins-jobs master and
removes possibility to update jenkins jobs from a refspec
(which is, honestly, a bad practice and should be avoided).

Change-Id: I07c20a46b6822d2c044370a27b8fe3cea56ccf7e
Closes-Bug: #1535607