Adds clarification note for project_id vs tenant_id

The client __init__ method takes both a project_id and tenant_id which is
rather confusing as in the Nova API these terms are used interchangeably. The
comment clarifies the difference between a project_id and tenant_id
when using novaclient.

For backwards compatibility reasons we can't really change the names (though for V3
perhaps we should in the future).

Change-Id: I569fe9318335c8d686153b0936205cb190e01ef1