Improve the cleanup code on enrollment handling error

[aleksmaus]

What is the problem this PR solves?

The cleanup code for enrollment errors is scattered in couple of places:

Router.handleEnroll->
	->EnrollerT.handleEnroll
		->EnrollerT.processRequest
			->EnrollerT._enroll
				->generateAccessApiKey + createFleetAgent(If Fail)->invalidateApiKey

	->writeResponse(If Fail) 
		-> wipeGhosts
			-> delete Agent document
			-> invalidateApiKey

This change consolidates the cleanup code through the rollback/cleanup wrapper.

How does this PR solve the problem?

Implemented a simple "rollback" wrapper that can have the "rollback/cleanup" cleanup functions added as needed. The Rollback is called on error which in turn calls the registered cleanup functions.

For example, the log when Fleet Server fails to create the agent record:

{"log.level":"error","service.name":"fleet-server","http.request.id":"01FMQW2DC9GXDDX3Z8S6YCTZ0M","mod":"enroll","fleet.enroll.apikey.id":"NxK5Jn0ByQ6Ip6IwWvny","error.message":"elastic fail 403:cluster_block_exception:index [.fleet-agents-7] blocked by: [FORBIDDEN/8/index write (api)];","@timestamp":"2021-11-17T21:15:31.625Z","message":"perform rollback on enrollment failure"}
{"log.level":"debug","service.name":"fleet-server","http.request.id":"01FMQW2DC9GXDDX3Z8S6YCTZ0M","mod":"enroll","name":"invalidate API key","@timestamp":"2021-11-17T21:15:31.625Z","message":"rollback function called"}
{"log.level":"info","service.name":"fleet-server","http.request.id":"01FMQW2DC9GXDDX3Z8S6YCTZ0M","mod":"enroll","fleet.enroll.apikey.id":"NxK5Jn0ByQ6Ip6IwWvny","fleet.apikey.id":"81fBL30BMo45IINiNv-r","dur":773.635427,"@timestamp":"2021-11-17T21:15:32.399Z","message":"invalidated apiKey"}
{"log.level":"debug","service.name":"fleet-server","http.request.id":"01FMQW2DC9GXDDX3Z8S6YCTZ0M","mod":"enroll","name":"invalidate API key","@timestamp":"2021-11-17T21:15:32.399Z","message":"rollback function succeeded"} 

So, now:

1. when the api key is created the api key invalidation function is registered with the Rollback
2. when the agent is created the delete agent function is registered with the Rollback
3. the deferred function in handleEnroll calls Rollback if there was an error during enrollment. the Rollback calls all the registered cleanup function in order to remove/invalidate the data that was created.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[mergify]

This pull request does not have a backport label. Could you fix it @aleksmaus? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v/d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2021-11-17T21:45:38.241+0000

• Duration: 9 min 3 sec

• Commit: 50a76bb

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🤖 GitHub comments

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[scunningham]

Seems ok. Not thrilled having to add the context to the router, but cleaner than previous implementation.