Add WeaveNet EXTRA_ARGS support

[ReillyTevera]

1. Describe IN DETAIL the feature/behavior/change you would like to see.
We have multiple kubernetes clusters that import all cluster logs to a logging service. Weave-net and weave-npc generate very large volumes of logs in our environment which causes us to use up our daily quote faster than desired.

Weave net supports modifying the log level through the --log-level= command line argument and allows you to pass in this argument via the EXTRA_ARGS environmental variable. It would be nice if we could support this feature in Kops through addition to the Weave networking spec.

Unfortunately weave-npc does not yet support this but I have added weaveworks/weave#3683 and once that or some variant of it is merged and a release of weave-net is cut we can add the environmental variable to weave-npc as well.

2. Feel free to provide a design supporting your feature request.

• I propose we add NetExtraArgs to the WeaveNetworkingSpec and when this is present add the EXTRA_ARGS to the weave-net container template
• NpcExtraArgs would be added at a later date to do the same with weave-npc

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[ReillyTevera]

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