Remove bucket name from object path prefix

[joao-zanutto]

Current implementation is duplicating the bucket name in the object path prefix:

This PR removes the bucket name from the prefix, which was being unnecessarily added.

As a suggestion: I was thinking further on this and it would be nice to have a default path prefix definition that includes the db host and db name to avoid confusion in case one just decides to dump multiple database dumps into the same bucket without defining a prefix.

Something like:
STORAGE_S3_PREFIX / DB_HOSTNAME-DB_NAME / DUMP_ID
Where it would be possible to set another config value (ie. STORAGE_S3_PATH or some other naming to avoid confusion) to override the DB_HOSTNAME-DB_NAME string.

This would result in:

• defining only the bucket would "just work" with logical separation of multiple databases in the same bucket
• defining a prefix would be ideal for the use case where one does not have a bucket exclusively for Greenmask dumps, but still doesn't need to "manually" define prefixes for each database
• possibility to override the the default path based on database/host if needed

[wwoytenko]

You've got the right thoughts. @flazzarini raised conversation regarding the storage path in #56

[wwoytenko]

Feel free to make a PR for this and consider the UX problem described in #56

[wwoytenko]

LGTM