Introduce env variable to optionally turn off storing on second bucket as not required in all environments

[pacospace]

Is your feature request related to a problem? Please describe.
cnv-prod environment for SLO reporter has only one bucket which should be also public (checking with Operate First about Hue/Superset). In stage environment we need to store in two buckets as thoth stage bucket is not public and cannot be reached by Hue/Superset used internally.

Describe the solution you'd like
Introduce env variable to optionally turn off storing on second bucket as not required in all environments

Describe alternatives you've considered
If main bucket name and public bucket name are the same, store only one one instead of setting another env variable.

Additional context
thoth-station/thoth-application#842

[pacospace]

Done already, THOTH_PUBLIC_CEPH_BUCKET not set will avoid trying to store in that bucket