Clarify "data" arg in create/update_image()

The argument "data" is used for passing it as raw data to Glance API,
that is different from the other arguments.
This patch separates the argument on the definition and adds some
docstring for linking to api-site.

Partially implements blueprint consistent-service-method-names

Change-Id: Ifeeceeb71c18d658ae6d5a0393150be3470264b3
1 file changed