Removes copy of output of 'nova help' from README

Removes the copy of the otuput of 'nova help' from the README.rst
file. It's out of date and given its not autogenerated is
rather prone to getting out of date again soon anyway. Just leave
the command that people need to run in order to print the help message.

Change-Id: I3cd069838302913bc69cc0ded5d632c7f8c3f408
Merge "Removes copy of output of 'nova help' from README"