One of the functionalities Nova provides is listing images.
This allows users to be able to consume the functionality,
without having to go through Glance.
(There are users who do not consume Glance).
Currently, there isn't a benchmark provided by rally that
does the above.
This commit adds a plugin that calls nova client list
method for Images, which will therefore be available
for benchmarking.
Includes functional and unit tests.
Change-Id: If11d623bd5c9bf7f8c723908cf9e1e332de0639d
bb9ecad478