Parse only lines with ":" from "show slave status"

https://github.com/racker/rackspace-monitoring-agent-plugins-contrib/issues/21

This is to fix the error that came up when trying to parse the following section of the "show slave status" command (i.e. lines without a colon):

      Replicate_Wild_Do_Table: 
  Replicate_Wild_Ignore_Table: 
                   Last_Errno: 1580
                   Last_Error: Error 'You cannot 'ALTER' a log table if logging is enabled' on query. Default database: 'mysql'. Query: 'ALTER TABLE slow_log
   MODIFY start_time TIMESTAMP NOT NULL,
   MODIFY user_host MEDIUMTEXT NOT NULL,