The exception message, when shade fails, will contain much more
specific information about the failure if the exception is treated
as a string. The 'message' attribute alone is usually not helpful.
The `os_network` module was incorrectly returning changed=False whether
or not the network was created. This commit makes the changed return
value useful.
With shade > 0.13.0, networks can be created that are externally
accessible. This adds a parameter for that.
Also, add RETURN documentation and 'if __name__' check around call
to main().
David Shrewsbury
Lars Kellogg-Stedman
Monty Taylor
Greg DeKoenigsberg