Add ORACLE_SYSTEM_USER parameter to system-app image

Not all the enterprise customers have access to Oracle System User, in porta https://github.com/3scale/porta/blob/master/lib/system/database.rb#L58
we have 'SYSTEM' username hardcoded we need to investigate if it is possible to substitute system with a generic user and make it available via an environment variable as already done for 'ORACLE_SYSTEM_PASSWORD'.