Added custom devstack repo and branch

We can use our own devstack with necessary fixes and additions.
Now the installer support handling of devstack repository and
branch.
Merge pull request #127 from sshnaidm/dev

Added custom devstack repo and branch
Support suctom branch and patchset testing

Now it's possible to test specific branch of any project and
specific patchset of any project, for example if you want to
test any of reviewed fixes for neutron.
Merge pull request #130 from sshnaidm/dev3

Support suctom branch and patchset testing