[query] Allow configuration of placement options

[notbdu]

What this PR does / why we need it:

Allows optional configuration of placement options inside of the m3query/coord config. Default behavior/options are retained as config options are optional.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

NONE

Does this PR require updating code package or user-facing documentation?:

NONE

[schallert]

LGTM if we're confident the config type change won't break existing config files.

[schallert]

Why change this from a pointer to directly embedded? I can't think of any reasons off the top of my head, but am wondering if there's some way this could break deserializing existing configs.

[notbdu]

I changed it since I wanted to use the default placement config inside of ClusterManagementConfiguration directly and didn't see any issues w/ the usage of this config w/in the code.

I don't think this will affect deserialization of existing configs? It's still optional in terms of the actual underlying yaml since I moved the pointer to be the actual Etcd config so prior behaviour is retained.

[codecov]

Codecov Report

Merging #3304 (5742617) into master (5742617) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3304   +/-   ##
=======================================
  Coverage    72.3%    72.3%           
=======================================
  Files        1099     1099           
  Lines      101740   101740           
=======================================
  Hits        73616    73616           
  Misses      23041    23041           
  Partials     5083     5083           

Flag	Coverage Δ
aggregator	76.4% <0.0%> (ø)
cluster	84.9% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	78.8% <0.0%> (ø)
m3em	74.4% <0.0%> (ø)
m3ninx	73.5% <0.0%> (ø)
metrics	19.9% <0.0%> (ø)
msg	74.2% <0.0%> (ø)
query	66.8% <0.0%> (ø)
x	80.7% <0.0%> (ø)

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