Follow st2 branching (master)

[arm4b]

Autodetect specific st2 branch depending on st2-packages branch.

What this change brings

• pushing to master of st2-packages repo will generate packages based on master of st2
• pushing to v1.3 of st2-packages repo will generate packages based on v1.3 of st2
• opening PR against master branch of st2-packages repo will use st2 master
• opening PR against v1.3 branch of st2-packages repo will use st2 v1.3
• opening PR against any other branch of st2-packages repo will use st2 master

This autodetect works for both Pull Requests and inside vX.Y and master branches of st2-packages repo.

Branch autodetection for PR is based on target branch:

armab wants to merge 1 commit into (--->) master from feature/branching

[arm4b]

As @enykeev correctly noted, autodetection won't work good enough with some PR cases:

you’re pushing it, it starts building, you’re creating a pr, you see test pass, you’re merging it, the merge fails

If you open PR and then merge more commits - it will autodetect target PR branch and will work well.
But it's error-prone way.