(MODULES-1287) Pass the backup credentials to 'SHOW DATABASES'

Pass the backup username and password to mysql when running SHOW DATABASES in
mysqlbackup.sh if `file_per_database` is true and `backupdatabases` is empty.
Merge pull request #559 from nhinds/file_per_database_username_and_password

(MODULES-1287) Pass the backup credentials to 'SHOW DATABASES'