feat(cloud): add shared-instance flag in limit superflag in alpha

[NamanJain8]

This PR adds a shared-instance flag to --limit superflag.
When set to true (false by default), it will:

• Restrict access to any of the ACL operations like Login, add/remove/update user from non-galaxy namespaces.
• Prevent the leaking of environment variables for minio and aws.

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This change is 

[jarifibrahim]

The PR looks good to me but we need tests. @NamanJain8 create a docker compose file with the cloud flag and then try to run login/create user, etc on that dgraph cluster.

[manishrjain]

some early comments. Haven't looked at the rest of the PR yet.

Reviewable status: 0 of 13 files reviewed, 4 unresolved discussions (waiting on @NamanJain8 and @vvbalaji-dgraph)

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dgraph/cmd/alpha/run.go, line 262 at r4 (raw file):

		String())

	flag.String("cloud", worker.CloudDefaults, z.NewSuperFlagHelp(worker.CloudDefaults).

how about put this in the "limit" flag?

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dgraph/cmd/alpha/run.go, line 264 at r4 (raw file):

	flag.String("cloud", worker.CloudDefaults, z.NewSuperFlagHelp(worker.CloudDefaults).
		Head("Dgraph cloud options").
		Flag("disable-non-galaxy",

probably don't need this. Unless you think the cloud flag would grow to many options. I reckon it's just for "shared-instance" users of the cloud.

So, perhaps limit=shared-instance=true, which would then automatically disable a bunch of things.

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testutil/multi_tenancy.go, line 347 at r4 (raw file):

func Export(t *testing.T, token *HttpToken, dest, accessKey, secretKey string) *GraphQLResponse {
	exportRequest := `mutation export($dst: String!, $f: String!, $access: String!, $secret: String!){

100 chars.

[jarifibrahim]

Get @ahsanbarkati's review as well.

[NamanJain8]

Reviewable status: 0 of 14 files reviewed, 3 unresolved discussions (waiting on @manishrjain and @vvbalaji-dgraph)

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dgraph/cmd/alpha/run.go, line 262 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

how about put this in the "limit" flag?

Done.

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dgraph/cmd/alpha/run.go, line 264 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

probably don't need this. Unless you think the cloud flag would grow to many options. I reckon it's just for "shared-instance" users of the cloud.

So, perhaps limit=shared-instance=true, which would then automatically disable a bunch of things.

Done.

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testutil/multi_tenancy.go, line 347 at r4 (raw file):

Previously, manishrjain (Manish R Jain) wrote…

100 chars.

Done.

I have addressed the above comments.