Allow MAX_SECOND_LATE to be user defined

On certain hosts I don't have the option to run cf-agent at 5 minute intervals so this will allow you to specify the number of seconds to allow before throwing an alert.

Also, the get_last_line_of_file function had references two references to CF_PROMISE_SUMMARY_FILE which can be replaced with filepath
`cf-isalive:149:80: E501 line too long (117 > 79 characters)`
line too long ;)