Make update_extra_routes use **kwargs & doc string

As we discussed on
http://lists.openstack.org/pipermail/openstack-dev/2015-July/068864.html
All http POST/PUT methods need to contain **kwargs as their arguments.

This patch makes update_extra_routes of network_client use **kwargs.
Also update doc string for the same.

Partially implements blueprint consistent-service-method-names

Change-Id: I4352ee72e4ccafd5a32cfb166bc692c5defe2c98
2 files changed