Replace deprecated MySQL system variable, fix #176
Replace deprecated MySQL system variable, fix #176

Replaces MySQL system variable `storage_engine` with
`default_storage_engine` because former is deprecated and was
removed in MySQL 5.7.5. Latter was added in 5.5.3 [1].

This commit also moves said setting into the django's DB setting
dictionary to de-clutter the code.

[1] https://dev.mysql.com/doc/refman/5.5/en/server-system-variables.html#sysvar_default_storage_engine

SensibilityTestbed/clearinghouse,SensibilityTestbed/clearinghouse,SensibilityTestbed/clearinghouse,SensibilityTestbed/clearinghouse