[V3] Rename get_credential to show_credential

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

Partially implements blueprint consistent-service-method-names

Change-Id: I07c4403e42d6f2c7617f455a8a4f292ff10e7059
3 files changed