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.


The system specific check was needed because right after I am doing a 'ubuntu' specific check for where the cmd defaults exist. I didn't want this change to fail un-wantedly for other distros. The above change/check worked just fine on my ubuntu system, and I tested it on non-ubuntu system as well to ensure it falls back to other behavior.