Set rpm-master-head as distgit branch for centos-master
Set rpm-master-head as distgit branch for centos-master

We will have some cases where we need to fix the centos-master
worker via a spec change that does not apply to centos-master-uc.
Both workers shared the same distgit branch, so let's create a new
one (rpm-master-head).

Change-Id: Ieda170b8abb8f70e16df41e615acd3e448bb78f4
Depends-On: I5276b90df1b81794514c95ee0347022c7133087a

rdo-infra/puppet-dlrn,rdo-infra/puppet-dlrn,javierpena/puppet-delorean,javierpena/puppet-delorean,rdo-infra/puppet-dlrn,javierpena/puppet-delorean