Fix 1539: Add --skip-repo option for Helm

If stablerepourl is not set, Helm will skip adding the stablerepo.

Depends on:
- https://osm.etsi.org/gerrit/#/c/osm/devops/+/10915



Change-Id: I11f10fdfba24ec274d463f12c07d7b0247b37068
Signed-off-by: David Garcia <david.garcia@canonical.com>