Make repo update to staging server configurable

Environment variable DO_UPDATE_STAGING_REPOSITORY is used to
select if repository is updated to staging server. Default
behavior is to always update staging server is env is not set.

Change-Id: I9840381d859536387bb9912e521d613c54fc4bfd
Reviewed-by: Antti Kokko <antti.kokko@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>