Add list-extensions to list of nova cli commands

This list-extensions command was added in Folsom.

from bug 1094955

There are other new flags of existing commands mentioned to add from
Folsom, but these require creating new examples, which may come later

A separate review will come for the grizzly features, so this one can
be cherrypicked for backport

Change-Id: Id41b9a4acfd7298d42a2030b8224f4d3b9b39ba6
Add grizzly nova client features to list

partially fixes bug 1094955

Adds the new commands in novaclient for grizzly to the list
of commands.

Examples for these should also be added.

Change-Id: Ie6c4bc8773655812a99c51731f2d406664f3c195
Merge "Add grizzly nova client features to list"
Merge "Add list-extensions to list of nova cli commands"