Provide add_qa_head_repo with arg:priority

At times it might be needed to change the priority of the QA repo,
as there might be - for instance - the need of letting zypper install
first from QA or from another repository. This is exactly the case for
blktests and fio package which is there in QA repo but also in SLE15

lib/repo_tools.pm

@@ -81,7 +81,10 @@ This repository *is* mandatory.
 
 =cut
 sub add_qa_head_repo {
-    zypper_ar(get_required_var('QA_HEAD_REPO'), name => 'qa-head', no_gpg_check => is_sle("<12") ? 0 : 1);
+    my (%args) = @_;
+    my $priority = $args{priority} // 0;
+
+    zypper_ar(get_required_var('QA_HEAD_REPO'), name => 'qa-head', priority => $priority, no_gpg_check => is_sle("<12") ? 0 : 1);
 }
 
 =head2 add_qa_web_repo