[GET /resources] methods should be "list_<resource name>s"
or "show_<resource name>", so this patch applies the rule
to server_group client.
This patch makes tempest-lib sync for the commit of Tempest
I75307176c2f032301da6227249070c42af54dd6e, but get_server_group has
been released in tempest-lib 1.0.0 already.
So it is difficult to remove it directly for backwards compatibility
and this patch adds warning message for clarifying deprecated method.
Partially implements blueprint consistent-service-method-names
Change-Id: I9e321aaf4791e8ff11e3aa114b41eb612db481ca
54d25c9615