JENKINS-24754 implemented support for Multi SCMs plugin, but still working with git, and support for using several git repositories with the normal git plugin.

Repositories are default named `origin`, `origin1` ....

and the pretested plugin needs configuration for integration branch (eg. `master`) and a named repository (eg. `origin`).

Default configuration is `master` and `origin`, thus using several git repositories and forgetting to chose names should work. It will just asume it is the first repository (which the git plugin names `origin` by default).

This is to not break all builds upon updating to next release (that is soon to be).
The choice is also made, as it is the current situation.