grenade pick master branch if requested stable doesn't exist

devstack-gate downloads master branch if specified stable branch doesn't exist in project [0].
As result each time when new stable branch is specified in devstack-gate [1]. All projects that do not cut this branch start perform testing from master ---> master.

An example, full log [2]:

2017-02-10 06:34:33.545 | + /home/jenkins/workspace/gate-grenade-dsvm-ironic-ubuntu-xenial/devstack-gate/functions.sh:git_has_branch:L292: grep remotes/origin/stable/ocata
2017-02-10 06:34:33.546 | + /home/jenkins/workspace/gate-grenade-dsvm-ironic-ubuntu-xenial/devstack-gate/functions.sh:git_has_branch:L295: return 1
2017-02-10 06:34:33.548 | ++ /home/jenkins/workspace/gate-grenade-dsvm-ironic-ubuntu-xenial/devstack-gate/functions.sh:setup_project:L505: echo refs/zuul/master/Z1853fe85095d4397975971eaf6001194
2017-02-10 06:34:33.549 | ++ /home/jenkins/workspace/gate-grenade-dsvm-ironic-ubuntu-xenial/devstack-gate/functions.sh:setup_project:L505: sed -e s,stable/ocata,master,

ironic-inspector http://logs.openstack.org/73/431673/1/check/gate-grenade-dsvm-ironic-inspector-ubuntu-xenial/5bb6c5d/logs/devstack-gate-setup-workspace-old.txt.gz#_2017-02-09_18_09_19_464

[0] https://github.com/openstack-infra/devstack-gate/blob/master/functions.sh#L502-L506
[1] https://github.com/openstack-infra/devstack-gate/commit/9c752b02fbd57c7021a7c9295bf4d68a0d1973a8
[2] http://logs.openstack.org/05/431205/3/check/gate-grenade-dsvm-ironic-ubuntu-xenial/6f969b0/logs/devstack-gate-setup-workspace-old.txt.gz#_2017-02-10_06_34_33_549