Kata agent and proxy fixes

[sameo]

Now that we can test all kata containers components with virtcontainers, here is my first round of fixes:

• Build and store a default gRPC socket.
• Move the gRPC client handling from kata_agent to kata_proxy.
• Misc fixes from Sebastien.

[jodh-intel]

Ping @sboeuf.

[sboeuf]

@jodh-intel I am gonna review this PR today !

[sameo]

@sboeuf I understand your concerns and it took me half a day to make the decision to go and implement the kata_proxy ops, for the following reasons:

• Make the current code consistent across proxy and agent implementations. Implementing everything inside kata_agent might be easier, but would make the current code base even more difficult to follow by having on one hand a cc_proxy+hyperstart_agent pattern that is different than the kata_proxy+kata_agent one.
• Implementing everything into kata_agent would not fix the overall problem we have with our agent+proxy implementation, it would only be an hybrid approach. I want to get a functional and consistent implementation working the broken proxy API that we have today, and then move to a reorganization of the agent+proxy architecture.
• No proxy could be supported with that approach and it could even be done through kata_proxy.go. If you use the kata_proxy.go code and a) do not start a proxy binary and b) Use the VSOCK URL as the proxy URL, you basically have a no_proxy implementation.

[sboeuf]

LGTM

[sboeuf]

PR merged since the Ubuntu 17.04 failure was not related to this PR but to the incapacity to reach some Ubuntu 17.04 repos.