[V3] Rename get_policy to show_policy

[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to policy client.

Partially implements blueprint consistent-service-method-names

Change-Id: Ic4cfe4a5ec5695d943f7dca9579c545285fb2719
3 files changed