don't fail on missing docker_tls_args
don't fail on missing docker_tls_args

instead just don't auto-detect TLS args in that case.

This makes `weave --local launch` work (again).

Fixes #1844.

tomwilkie/weave,rade/weave,n054/weave,tomwilkie/weave,rade/weave,tomwilkie/weave,paulbellamy/weave,n054/weave,dpw/weave,n054/weave,tomwilkie/weave,paulbellamy/weave,n054/weave,tomwilkie/weave,weaveworks/weave,paulbellamy/weave,n054/weave,weaveworks/weave,weaveworks/weave,paulbellamy/weave,rade/weave,dpw/weave,weaveworks/weave,dpw/weave,dpw/weave,rade/weave,rade/weave,dpw/weave