*: make discovery testable

[gyuho]

Preliminary work towards #52.

Goal is having good test coverage on current code base, so that we can safely migrate to etcd v3. This does not change any client-facing interface (etcd v2). Highlighted changes are:

• Update vendors with latest etcd release, sync other upstream dependencies
• Remove test files in vendor
• Use Go 1.8.3 (required for integration tests with latest etcd)
  • Go 1.6<= won't build, Go 1.7<= will build but fail in CI
• Change Go version to 1.8.3 in Dockerfile
• Share states between handlers with context, for integration testing (avoids global vars)
• Remove pkg/lockstring, instead have mutex inside shared states between handlers
• Add handler integration tests with embedded etcd (available since v3.1)
• Add e2e tests with actual etcd cluster and discovery.etcd.io builds
• Set up CIs

In next PR, I will add v3 API option, with migration e2e tests.

[gyuho]

@bison @xiang90 Please review. This doesn't change any client-facing API.
I disabled quay build trigger, to be safe.

[gyuho]

@bison

I just discussed with @xiang90, and figured out v3 implementation of discovery service cannot happen anytime soon. Problem is that current etcd code base heavily relies on v2 API https://github.com/coreos/etcd/blob/master/discovery/discovery.go#L143.

Minimum requirements to deprecate v2 API in public discovery service are:

1. Support v3 storage backend in https://github.com/coreos/discovery.etcd.io (this is fairly easy to do, without any breaking change)
2. Support v3 API in https://github.com/coreos/etcd/blob/master/discovery (this is much more complicated, and need wait until next release cycle)

Even if we implement them all, etcd old versions (v3.x<=) will still write v2 data to discovery service servers. So the true deprecation of v2 storage backend in discovery service can only happen after v2 deprecation in etcd side (planned for v4).

I still want to merge this PR, for v3 API reference implementation and for v2 data purge scripts.

After this PR, I will add some docs on purge operations.