Improve deployment stability

[pandatix]

In this PR I do not focus on improving features nor configurations. I rather improve the deployment stability through:

• a new /healthcheck endpoint that returns the current status of Chall-Manager and the underlying etcd cluster if configured. This enable the janitor to not start until CM is ready, so avoid useless restarts.
• a new Pulumi stack configuration janitor.mode to define whether to use the janitor in cron or ticker mode ; and a janitor.ticker to configure the tick frequency.
• a new Pulumi check for the services.ChallManager that looks for inconsistencies in the configuration, like replicas without an etcd cluster (no consensus can be established thus could not guarantee consistency).
• better testing strategies with the use of both assert and require rather than only dealing with assertions when requirements must be fulfilled

Solves #402

[coveralls]

Pull Request Test Coverage Report for Build 12968475748

Details

• 0 of 75 (0.0%) changed or added relevant lines in 4 files are covered.
• 29 unchanged lines in 1 file lost coverage.
• Overall coverage remained the same at 0.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server/server.go	0	1	0.0%
cmd/chall-manager/main.go	0	3	0.0%
server/healthcheck.go	0	34	0.0%
cmd/chall-manager-janitor/main.go	0	37	0.0%

Files with Coverage Reduction	New Missed Lines	%
cmd/chall-manager-janitor/main.go	29	0.0%

Totals
Change from base Build 12965550824:	0.0%
Covered Lines:	0
Relevant Lines:	4902

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💛 - Coveralls

[NicoFgrx]

LGTM