enforce retention_days is int type

[MeredithAnya]

If the retention_days provided is not an int we will just use the DEFAULT_RETENTION_DAYS. Should fix #2780.

[codecov-commenter]

Codecov Report

Merging #2809 (7c72124) into master (02dbd05) will decrease coverage by 0.35%.
The diff coverage is 100.00%.

❗ Current head 7c72124 differs from pull request most recent head e4782d1. Consider uploading reports for the commit e4782d1 to get more accurate results

@@            Coverage Diff             @@
##           master    #2809      +/-   ##
==========================================
- Coverage   92.83%   92.47%   -0.36%     
==========================================
  Files         611      612       +1     
  Lines       28639    28662      +23     
==========================================
- Hits        26586    26505      -81     
- Misses       2053     2157     +104     

Impacted Files	Coverage Δ
snuba/settings/settings_distributed.py	100.00% <ø> (ø)
snuba/clusters/storage_sets.py	100.00% <100.00%> (ø)
snuba/datasets/events_format.py	94.66% <100.00%> (ø)
snuba/migrations/groups.py	94.79% <100.00%> (+0.34%)	⬆️
...igrations/snuba_migrations/replays/0001_replays.py	100.00% <100.00%> (ø)
snuba/settings/__init__.py	94.59% <100.00%> (ø)
...nsactions_onpremise_fix_orderby_and_partitionby.py	32.00% <0.00%> (-52.00%)	⬇️
...ts/0010_groupedmessages_onpremise_compatibility.py	51.11% <0.00%> (-48.89%)	⬇️
tests/migrations/test_runner_individual.py	52.74% <0.00%> (-40.66%)	⬇️
...ns/snuba_migrations/events/0014_backfill_errors.py	92.72% <0.00%> (-3.64%)	⬇️
... and 2 more

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