Use polymorphism for database backup

[acatton]

(I'm creating these issues as note to myself or anybody who wants to work on django-backupdb)

• django-backupdb/backupdb/management/commands/backupdb.py

  Line 81 in e705bb1

  backup_func = backup_config['backup_func']

• django-backupdb/backupdb/management/commands/backupdb.py

  Line 87 in e705bb1

  if backup_func is do_postgresql_backup:

• django-backupdb/backupdb_utils/settings.py

  Line 13 in e705bb1

  BACKUP_CONFIG = {

• django-backupdb/backupdb_utils/commands.py

  Line 59 in e705bb1

  def get_mysql_args(db_config):

This is kind of nasty. It should use inheritance/polymorphism, and do something like (just the idea):

backend_class = backends[backend_name]
backend = backend_class(**credentials)
backend.do_backup()