tentative support for an external controlling coordinator

[xmcqueen]

This allows the operator to startup in db-only mode (no colocated coordinator). Uses an externally managed coordinator. Start the db with no "coordinator" section in the configmap.

Pass in the labels of the externally controlled coordinator's service endpoint.

spec: . . . externalCoordinatorSelector: app: coordinator . . .

The db-only cluster will boot up using the external coordinator.

[codecov]

Codecov Report

Merging #208 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   75.28%   75.30%   +0.02%     
==========================================
  Files          30       30              
  Lines        2096     2098       +2     
==========================================
+ Hits         1578     1580       +2     
  Misses        391      391              
  Partials      127      127              

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[schallert]

Thanks for the contribution, @xmcqueen!