add some pre-req checks for weave launch

Create a 'global params' check function that can do prerequisite checks for various things that a 'weave launch' requires. For one, I know it requires is 'docker0' bridge i.e. docker be launched with default bridge. 
This pull-req fixes  the issue mentioned in #189

Tested this with and without docker networking installed.


What is the actual pre-requisite here -- does `weave` actually depend on the docker bridge? Is it possible to run docker with a differently-named bridge?