Make key container creation idempotent

[raildo]

Container creation returns a 409 Conflict if the container
already exists, but since the request doesn't take any
additional parameters, there's really no conflict.
Update the API return, if the container exists, to 200 OK.
Users could still distinguish whether the container
already existed by checking for 200 OK vs 201 Created.

Signed-off-by: Raildo Mascena <rmascena@redhat.com>
Closes: #206

[simo5]

I do not get why lint failed ...

[codecov-io]

Codecov Report

Merging #207 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #207      +/-   ##
==========================================
+ Coverage   69.33%   69.34%   +<.01%     
==========================================
  Files          31       31              
  Lines        3163     3164       +1     
  Branches      346      346              
==========================================
+ Hits         2193     2194       +1     
  Misses        852      852              
  Partials      118      118

Impacted Files	Coverage Δ
src/custodia/client.py	79.6% <ø> (ø)	:arrow_up:
src/custodia/secrets.py	66.66% <100%> (+0.1%)	:arrow_up:
tests/test_secrets.py	98.88% <100%> (ø)	:arrow_up:

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[raildo]

I fixed it in another PR, I push to this branch too, not it will works :)