Feature: The role will determine the default database type used by postfix
and will export that value as the variable `postfix_default_database_type`

Reason: Users need to be able to set configuration parameter values
based on the database type.

Result: Users can set configuration parameter values based on the
default database type.

NOTE: Using postfix_default_database_type in a configuration parameter
value is not supported on Ansible 2.9 due to the way 2.9 does variable
evaluation.

Signed-off-by: Rich Megginson <rmeggins@redhat.com>