[tools] CLI tool to simplify some tasks, and eliminate the curl commands from the docs

[xmcqueen]

What this PR does / why we need it:

I'm not aware of a ticket asking for this, but I've seen in the chat's an interest in tooling that will allow the docs to look cleaner by getting rid of the curl commands.

I'm working with m3db a bit and found I wanted some of this functionality as a tool, so I made it. I thought it would be a good fit here.

# show help
m3ctl -h
# create a database
m3ctl db create -f ./database/examples/devel.yaml
# list namespaces
m3ctl ns
# delete a namespace
m3ctl ns delete -name default
# list placements
m3ctl pl
# point to some remote and list namespaces
m3ctl -endpoint http://localhost:7201 ns
# check the namespaces in a kubernetes cluster
# first setup a tunnel via kubectl port-forward ... 7201 
m3ctl -endpoint http://localhost:7201 ns
# list the ids of the placements
m3ctl -endpoint http://localhost:7201 pl | jq .placement.instances[].id

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?:

If the tool is accepted it would be good to integrate it into the operational-guides pages, replacing the curl commands.

[pavelnikolov]

Awesome idea 👏

[codecov]

Codecov Report

Merging #2097 into master will decrease coverage by 0.3%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2097     +/-   ##
========================================
- Coverage    72.4%     72%   -0.4%     
========================================
  Files        1007    1003      -4     
  Lines       86460   86415     -45     
========================================
- Hits        62614   62261    -353     
- Misses      19676   19955    +279     
- Partials     4170    4199     +29

Flag	Coverage Δ
#aggregator	82% <ø> (ø)	⬆️
#cluster	85.4% <ø> (-0.2%)	⬇️
#collector	82.8% <ø> (+18%)	⬆️
#dbnode	79.7% <ø> (-0.1%)	⬇️
#m3em	74.4% <ø> (+1.1%)	⬆️
#m3ninx	74.1% <ø> (+0.1%)	⬆️
#m3nsch	51.1% <ø> (ø)	⬆️
#metrics	17.6% <ø> (-0.1%)	⬇️
#msg	74.7% <ø> (-0.2%)	⬇️
#query	68.5% <ø> (ø)	⬆️
#x	75.4% <ø> (-7.8%)	⬇️

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[robskillington]

LGTM