Call lf-activate-venv() to create venv to install jenkins-jobs. Remove
references to jjb-cleanup.sh & jjb-install.sh scripts. Some jobs still
depend on them, so they have to remain for now.
Issue: RELENG-2557
Change-Id: Ib6b8a6e65113a874994a447cb628ca08dc9604c0 Signed-off-by: Tim Johnson <tijohnson@linuxfoundation.org>