[V3] Rename get_group to show_group

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

Partially implements blueprint consistent-service-method-names

Change-Id: Ia6c83f9bad2684c61e694e52244ea54c98d9d070
3 files changed