CFN-564(DBClusterParameterGroup) - Fix DBClusterParameterGroup not resetting parameters

[angusy29]

Description of changes:

DBClusterParameterGroup was not correctly resetting parameters.

The bug:

Eg. Try creating a DBClusterParameterGroup with:

        require_secure_transport: "ON"
        lower_case_table_names: "1"
        collation_server: "utf8_bin"
        time_zone: "US/Eastern"
        character_set_server: "utf8"
        server_audit_events: "CONNECT,QUERY"
        server_audit_logging: "1"

Then performing an update with:

        lower_case_table_names: "1"
        collation_server: "utf8_bin"
        time_zone: "US/Eastern"
        character_set_server: "utf8"
        server_audit_events: "CONNECT,QUERY"
        server_audit_logging: "1"

The parameter require_secure_transport does not get reset to its default value.

DBParameterGroup however, does exhibit the correct reset behaviour and resets the parameter if it is removed.

This CR ensures the DBClusterParameterGroup behaves like the DBParameterGroup resource when removing parameters.

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