Add option for TLS enabled etcd server connections.

[PrismaPhonic]

This PR adds the option for TLS enabled etcd server connections, and includes a test to confirm that when an etcd instance has been started with key, and cert, that a client setup with the correct TLS info can connect, put a value on a key, and retrieve that value off the key.

[enisoc]

We shouldn't be closing an unexported field from the server. Cleaning up internal resources is the job of the owning object (e.g. this should happen inside server.Close() if necessary).

[PrismaPhonic]

Noted, will fix. Is there some equivalent of Drop for golang? (A way to provide clean-up instructions that are called automatically when the GC decides to clean an object up?)

[PrismaPhonic]

eafeeb4.

[enisoc]

There is a way to do that in Go, but we do not speak of it because it must never be used. You didn't hear this from me. I was never here.

[enisoc]

LGTM

comments have been addressed

[enisoc]

The unit test is failing:

--- FAIL: TestEtcd2TLS (6.08s)
##[error]    server_test.go:150: newCellClient(https://localhost:6700) failed: dial tcp 127.0.0.1:6700: connect: connection refused
FAIL
FAIL	vitess.io/vitess/go/vt/topo/etcd2topo	9.431s

I think you need to push the clientv3.New() call into the retry loop since the server might not be ready yet. Also it looks like you don't yet Close() this test client.