oradb_manage_db: Define defaults for oracle_db_mem_totalmb

[Rendanic]

The memory-settings in oracle_databases should be easier to handle for new users.
The oracle_db_mem_totalmb is only used during initial database creation and often overwritten by init.ora parameters.

Solution:

• assert that oracle_db_mem_totalmb is NOT defined and sga_target is defined in ass_odb.init_parameters
• assert that oracle_db_mem_totalmb is defined and sga_target is NOT defined in ass_odb.init_parameters

This has been solved with #414