contrail-charms: add option to disable external content

Contrail networking 4.1.0 with Canonical Openstack/juju charms.

Currently, external internet access is needed when installing Contrail via juju charms. This is not possibkle in most telco cloud environments. THerefore it is required to be able to install contrail without external connectivity. (Internal repo is typically available which should have all necesaary packages).

For example, the following external link shall not be hardcoded: https://github.com/Juniper/contrail-charms/search?utf8=%E2%9C%93&q=dockerproject.org&type=

or there should be an option to switch repo addition off, e.g. make this optional:
- https://github.com/Juniper/contrail-charms/blob/2db530b262958afaadf580da0c049bb36acc6aeb/contrail-analyticsdb/hooks/contrail_analyticsdb_hooks.py#L58
- https://github.com/Juniper/contrail-charms/blob/e85d3e1c127a2270eb04e62673a7887636439be4/contrail-analytics/hooks/contrail_analytics_hooks.py#L53
- https://github.com/Juniper/contrail-charms/blob/e85d3e1c127a2270eb04e62673a7887636439be4/contrail-controller/hooks/contrail_controller_hooks.py#L73