Fix 1539: Helm v2.17.0 and set default value for stablerepourl

The --skip-repo option was not available. The version v2.17.0 is the
latest v2 version available.

Set https://charts.helm.sh/stable

 as the default stablerepourl value.

Change-Id: Ifdd85f0f891a705f56cdd50c6e4dbf95820d706f
Signed-off-by: David Garcia <david.garcia@canonical.com>