[dist/ci] Restructure CI jobs to allow to build several git branch.

[dist/ci] Restructure CI jobs to allow to build several git branch.

The Jenkins git plugin isn't flexible enough to trigger build jobs
for several git branches, thus one has to create a job per branch to
test. Thus the obs_testsuite_api and obs_common jobs templates where
merged. Per default, the obs_testsuite_api job template tests the git
master branch. If you want to test the git 2.3 branch, copy this job
template and replace all occurences of 'master' with '2.3' :-)