mgr/dashboard: clean-up python unit tests
Currently Python unit tests (actually doctests) bring in dependencies for other packages (rados, rbd, cephfs). Most of them aren't actually used at doctests, but cause
By removing those dependencies, dashboard python unit-tests (240) now can be quickly run (<40 s) in python-only docker environments (e.g.: travis-ci, or CircleCI), without needing extra packages (python-rados, or RBD/CephFS python bindings).