`--sql-db` does not take effect

If `--sql-db` is given, it does not do anything. Users trying to backup with `knife ec backup` that have a DB name differing from the DB username are unable to use this tool. This was missed in a refactor a [long time ago](https://github.com/chef/knife-ec-backup/commit/8c87580314ab8a5ca52fbbdc81cb516cb9a73304).

PR with fix: https://github.com/chef/knife-ec-backup/pull/182