storage: Move flag out of the package to main.go

[pav-kv]

Move the flag so that it is always initialised after storage providers are imported.

$ go run main.go --help
...
  -storage_system string
        Storage system to use. One of: [cloud_spanner mysql] (default "mysql")
...

Similar to issue #2026.

Checklist

• I have updated the CHANGELOG.
  • Adjust the draft version number according to semantic versioning rules.
• I have updated documentation accordingly (including the feature implementation matrix).

[pav-kv]

PTAL

[AlCutter]

storage/provider_test.go:80:8: undefined: providers

[pav-kv]

Fixed the tests

[codecov]

Codecov Report

Merging #2287 (f53436d) into master (219fef4) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2287   +/-   ##
=======================================
  Coverage   65.00%   65.00%           
=======================================
  Files         118      118           
  Lines        8903     8902    -1     
=======================================
  Hits         5787     5787           
+ Misses       2456     2455    -1     
  Partials      660      660           

Impacted Files	Coverage Δ
storage/provider.go	88.88% <ø> (+4.67%)	⬆️

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