Fix bootstrapping on non-init servers

[brandond]

Proposed Changes

• Serve HTTP bootstrap data from datastore before disk
  Fixes issue where CA rotation would fail on servers with join URL set due to using old data from disk on other server.
• Control-plane-only nodes try bootstrapping via etcd load-balancer if they have client certs, to avoid a dependency on the server URL
  Fixes issue where control-plane-only nodes could not come up if the server they were joined against is not up, despite other servers being available.

Types of Changes

bugfix

Verification

See linked issue

Testing

Linked Issues

• rotate-ca doesn't work if server is set in config when rotating #11233
• control-plane-only nodes cannot start if join server is unreachable #11333

User-Facing Change

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 15.52795% with 136 lines in your changes missing coverage. Please review.

Project coverage is 44.92%. Comparing base (5894af3) to head (365131f).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/cluster/bootstrap.go	11.26%	59 Missing and 4 partials ⚠️
pkg/cluster/cluster.go	0.00%	30 Missing ⚠️
pkg/cluster/storage.go	25.00%	15 Missing ⚠️
pkg/clientaccess/token.go	38.88%	10 Missing and 1 partial ⚠️
pkg/etcd/etcd.go	0.00%	8 Missing ⚠️
pkg/daemons/control/server.go	33.33%	2 Missing and 4 partials ⚠️
pkg/server/handlers/secrets-encrypt.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11844      +/-   ##
==========================================
- Coverage   47.54%   44.92%   -2.62%     
==========================================
  Files         190      190              
  Lines       19022    19094      +72     
==========================================
- Hits         9044     8578     -466     
- Misses       8690     9274     +584     
+ Partials     1288     1242      -46     

Flag	Coverage Δ
e2etests	35.89% <11.18%> (-5.48%)	⬇️
inttests	35.32% <13.04%> (-0.09%)	⬇️
unittests	16.80% <3.72%> (-0.07%)	⬇️

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