[FIXED JENKINS-18237] Fix matrix multiple scm builds

This fixes a bug that causes Matrix Builds to fail when using two or more Mercurial repositories (via the multiple-scms plugin).

Previously, each SCM tried to checkout the changeset from the first Mercurial repository (which caused the build to fail, obviously).  This checks for the presence of the multiple-scms plugin and, if the build uses it, looks up the parent changeset using multiple-scms.

This adds `multiple-scms` as an optional dependency (so I can change behavior when it is used).  Since it is now in the POM, I removed the *simplified* MultiSCM test classes.  I also modified the MatrixBuild tests to fail faster when builds fail (added an await timeout).

[JENKINS-18237](https://issues.jenkins-ci.org/browse/JENKINS-18237)