Add tenant list API

Add client support for retrieving a list of tenants.

Signed-off-by: Ryu Ishimoto <ryu@midokura.com>
Add tenant list support in the CLI

Add both interactive and single command support to list tenant
IDs.

Signed-off-by: Ryu Ishimoto <ryu@midokura.com>
Merge pull request #35 from ryu25ish/tenant_api

Add tenant list API