In _prep_server_detail(), power_state is not formatted
by _format_servers_list_power_state(). So when executing
"server show" or "server create", the power state is
represented by number. This patch map the numbers to
meanful strings.
This patch also adds power_state attribute to FakeServer,
and improves unit tests for this attribute.
Change-Id: I2ec674327de4e5133b8712ba6bb53fa5ce55e3f4