msp handler: be more robust in is_valid_source
msp handler: be more robust in is_valid_source

RedHat-Eng-PGM/schedules-tools,RedHat-Eng-PGM/python-schedules-tools,RedHat-Eng-PGM/python-schedules-tools,RedHat-Eng-PGM/schedules-tools,RedHat-Eng-PGM/python-schedules-tools,RedHat-Eng-PGM/schedules-tools