There is no such stanza in Upstart. "limit cpu" stanza will limit the maximum time (in seconds) process is allowed to be executed on CPU. During this allowance, it can take 100% of the CPU.
See man setrlimit(2): http://linux.die.net/man/2/setrlimit
CPU time limit in seconds. When the process reaches the soft limit, it is sent a SIGXCPU signal. The default action for this signal
is to terminate the process. However, the signal can be caught, and
the handler can return control to the main program. If the process
continues to consume CPU time, it will be sent SIGXCPU once per second
until the hard limit is reached, at which time it is sent SIGKILL.
(This latter point describes Linux behavior. Implementations vary in
how they treat processes which continue to consume CPU time after
reaching the soft limit. Portable applications that need to catch this
signal should perform an orderly termination upon first receipt of