Add option to apply enterprise license at zero's startup.

[martinmr]

GitHub Issue or Jira number.

https://dgraph.atlassian.net/browse/DGRAPH-1218

Affected releases.

20.03.1

Changelog tags.

Added

Please indicate if this is a breaking change.

No

Please indicate if this is an enterprise feature.

Yes

Please indicate if documentation needs to be updated.

Yes. The changes needed are already included in this PR.

Please indicate if end to end testing is needed.

No

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This change is 

Docs Preview:

[danielmai]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, 1 of 1 files at r3, 1 of 1 files at r4.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @manishrjain, @martinmr, and @MichaelJCompton)

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dgraph/cmd/zero/license.go, line 41 at r4 (raw file):

}

func (st *state) applyLicenceFile(path string) error {

applyLicenseFile (spelling)

[martinmr]

Reviewable status: 2 of 4 files reviewed, 3 unresolved discussions (waiting on @danielmai, @golangcibot, @manishrjain, and @MichaelJCompton)

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dgraph/cmd/zero/license.go, line 41 at r4 (raw file):

Previously, danielmai (Daniel Mai) wrote…

applyLicenseFile (spelling)

Done.

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dgraph/cmd/zero/license_ee.go, line 138 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

	content, err := ioutil.ReadFile(path)
	if err != nil {

Done.

---

dgraph/cmd/zero/license_ee.go, line 138 at r2 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

	content, err := ioutil.ReadFile(path)
	if err != nil {

Done.

[danielmai]

Please cherry-pick to release/v20.03 too.

Reviewed 2 of 2 files at r5.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @danielmai, @golangcibot, @manishrjain, and @MichaelJCompton)

[martinmr]

Cherry-picked it into the release branch.