[feature] Add optional network-topology module

Created by: nemesisdesign

When the first release of openwisp-network-topology is ready, add a way to install, configure and upgrade the module.

@rohithasrk will help me implementing this feature. @leonardomaccari and @mikysal78 are also interested in using this feature.

It would be great to figure out a general way to extend ansible-openwisp2 with additional roles, that way we could allow adding extra features to ansible-openwisp2 without complicating it too much: additional features like the network-topology and the freeradius modules would then become additional ansible roles that depend on ansible-openwisp2, but this is just an early-stage idea.