Running microstack.remove will remove the br-ex virtual bridge device,
then uninstall MicroStack.
We do this because we can't use ovs-ctl to remove the bridge as part
of a remove hook, as the Open vSwitch daemons are not running at that
point. The microstack.remove command gives operators a way to cleanly
uninstall the snap, without needing to reboot to get rid of br-ex.
Added test exercising the code to test_basic.py.
Rerranged entry points a bit (moved some things into main.py) to make
code sharing easier, and to prevent a proliferation of entry point
scripts in our root dir.