OPENDEV: Add additional parameters to SCM module

The projects should have control about their workspace.
Following parmaters are introduced for git-scm and git-scm-gerrit
to control what should be done with git repository when the
jenkins job is executed:
- wipe_workspace: Wipe out repository and force clone (default: true)
- clean_before: Clean up the workspace before every checkout
  by deleting all untracked files and directories (default: false)

Change-Id: I6216334251d9af8c0c6b9818b2ef3280d6196ef5