Set sane defaults for required conf params in trove/common/cfg.py

The following conf options are required and should have their defaults
set to sane values which make sense for the reference implementation:
ignore_dbs, ignore_users, admin_roles, and api_extensions_path

Change-Id: I2d78e9a06cb56832297d38a8fc4608964bca6976
Fixes: bug 1226496
Merge "Set sane defaults for required conf params in trove/common/cfg.py"