fix(server): fix client certificate verification

In d0c4916fe9b2afaa273a2a7bc9782321a866ab9f the TLS CA Certificate
verification broke.

This was bisected using the following basic test:

```
./bin/etcd -f -name machine0 -data-dir machine0 -ca-file=/tmp/ca/ca.crt -cert-file=/tmp/ca/server.crt -key-file=/tmp/ca/server.key.insecure
```

And in another window doing

```
curl --key /tmp/ca/server2.key.insecure  --cert /tmp/ca/server2.crt -k -L https://127.0.0.1:4001/v2/keys/foo -XPUT -d value=bar -v
```

Before merging this PR there are a few things that need to be fixed up:

1) Tests for client certs both positive and negative
2) Refactor (or at least documentation of) the TLSConfig types
doc(server): some basic docs on the tls_config object

This should be refactored but something to remember while refactoring.
test(TLS): Add test coverage for etcd TLS
Merge pull request #1 from bcwaldon/fixup-server-tls-client-config

test(TLS): Add test coverage for etcd TLS
Merge pull request #519 from philips/fixup-server-tls-client-config

fix(server): fix client certificate verification