[pip] Avoid installing more recent versions of package unintentionally

Created by: nemesisdesign

I realized only now that with https://github.com/openwisp/ansible-openwisp2/commit/3151bd0e87211e7a7340158eefa80f50d388a803 we run the risk of installing more versions of dependencies that are not compatible with openwisp-controller unintentionally.

Suppose we release a new major version of one of the dependencies which is out of the range specified in the requirements of openwisp-controller, what would happen? It would be installed and OpenWISP 2 would break. That would affect negatively every user trying to install/upgrade their instance.

So the solution is to reintroduce the variables with the _dev suffix, and install the modules specified in the variables with the _pip suffix only if the corresponding _dev is enabled, and each _pip suffix variable should point by default to each repo's github tarball of master.