feat(dgraph): enabling TLS config in http zero

[aman-bansal]

Enabling TLS based encryption in Dgraph zero.
Fixes https://dgraph.atlassian.net/browse/DGRAPH-1366. Based on discussion here https://discuss.dgraph.io/t/enabling-tls-configuration-for-internal-ports-making-dgraph-more-secure/10860/2
The same variables are used which needs to be defined for alpha tls configuration

Things to consider:

1. With TLS enabled, tls_enabled_route has to be defined.
2. If defined all routes will be accessible with TLS. But ones mentioned in --tls_enabled_route will not be accessible via HTTP.

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This change is 

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[codelingo]

LGTM. 15 rules errored during the review.

[pawanrawal]

Reviewable status: 0 of 10 files reviewed, 2 unresolved discussions (waiting on @aman-bansal, @manishrjain, and @vvbalaji-dgraph)

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dgraph/cmd/zero/http.go, line 246 at r1 (raw file):

}

func (st *state) serveHTTP(l net.Listener) {

should be renamed as it serves both HTTP and HTTPS

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dgraph/cmd/zero/http.go, line 278 at r1 (raw file):

func startServers(m cmux.CMux) {
	// if tls enabled http rule will check that the requested route should not be part of tls_enabled_routes

The naming needs to be fixed here.

[parasssh]

-all tests for the combination of tls config (on/off) and enabled routes (0 , all, some).

• bulk loader system test with and without TLS.
• System test with alpha, zero and using Ratel. Full TLS enabled.

[aman-bansal]

Both bulk live or other clients connect with GRPC port of zero. This change doesn't affect any of those. We already have tests running to check those features.

For ratel, I have tested this locally. But couldn't find anything that tests end to end integration of zero, alpha, ratel.

[aman-bansal]

I changed the enable flag to disable because there are many endpoints like /debug which isn't initiated directly. We can introduce some side effects with negation logic. Also this align more with manish comment

Another thing to do would be to have a flag, which can tell the HTTP endpoints which can be plain-text. For e.g., “/health, /debug”, etc. That way, a very secure usage can keep them empty. But, selectively open some to easier `debugging.

[pawanrawal]

Reviewed 4 of 10 files at r1, 4 of 6 files at r2, 2 of 2 files at r3.
Reviewable status: all files reviewed, 4 unresolved discussions (waiting on @aman-bansal, @manishrjain, and @vvbalaji-dgraph)

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dgraph/cmd/zero/run.go, line 97 at r3 (raw file):

	flag.Bool("tls_use_system_ca", true, "Include System CA into CA Certs.")
	flag.String("tls_client_auth", "VERIFYIFGIVEN", "Enable TLS client authentication")
	flag.String("tls_disabled_route", "", "comma separated zero endpoint which will disabled from TLS encryption."+

which will be ....

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tlstest/zero_https/no_tls/no_tls_test.go, line 58 at r3 (raw file):

	defer func() { _ = do.Body.Close() }()
	body, err := ioutil.ReadAll(do.Body)
	if err != nil {

require.NoError(t, err)