Add 'flavor-list --all' admin switch

Nova gives admins public flavors and flavors from their own projects
only by default. For flavor management, admins need to see all flavors
regardless of access.

Changes:
- Adds an 'is_public' argument to flavors.list() that mirrors the
  Nova API. is_public=None can be used to see all flavors.
- Adds an --all switch to flavor-list in the CLI for use by admins
  when all flavors are wanted.

Fixes bug 1172179.

Change-Id: I915cd2d8266cb6e32c80691a6ff27d8a23488c98