[dtest] ns update/delete api

[gediminasgu]

What this PR does / why we need it:
Adds namespace update/delete methods to test API to allow us to change namespace settings during the test.

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

[linasm]

LGTM, some minor nits inline.

[codecov]

Codecov Report

Merging #3344 (8ece790) into master (8ece790) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3344   +/-   ##
=======================================
  Coverage    72.6%    72.6%           
=======================================
  Files        1098     1098           
  Lines      103388   103388           
=======================================
  Hits        75096    75096           
  Misses      23145    23145           
  Partials     5147     5147           

Flag	Coverage Δ
aggregator	76.8% <0.0%> (ø)
cluster	84.9% <0.0%> (ø)
collector	84.3% <0.0%> (ø)
dbnode	79.0% <0.0%> (ø)
m3em	74.4% <0.0%> (ø)
m3ninx	73.5% <0.0%> (ø)
metrics	19.8% <0.0%> (ø)
msg	74.5% <0.0%> (ø)
query	67.3% <0.0%> (ø)
x	80.5% <0.0%> (ø)

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