Do not use ssh for checkout.
Improve ability to tests externals PRs.

- Allow to specify a branch, rather than a PR.
- Allow to specify a CMSDIST PR, rather than using the
  default CMSDIST branch for the requested release series.
- Allow specifying a diffenent package as final target of
  the build, e.g. cmssw-tool-conf
Merge pull request #196 from ktf/add-support-for-extra-prs

Improve ability to tests externals PRs.