Inconsistent behaviour for update_check_definition wtr to last_modified_by attribute

[ofreshy]

When updating check definition via the python client (but not via the command line cli), the 'last_modified_by' field is required. The python docs only states that the owing team is required :

• http://zmon.readthedocs.io/en/latest/developer/zmon-python-client.html#zmon_cli.client.Zmon.update_check_definition

Proposed Solutions:

• Update the docs to make it required by the python client
• Attempt to read the user name if not provided (Similar to cmd line client)

I believe that the second option is better as more consistent with the cmd line client.