restapi: Fix optional parameters of add template

The operation to add a template accepts the cluster name and id as
optional parameters, but they aren't documented in the metadata, thus
they aren't available in the CLI.

Change-Id: I612da88a8de940f635cf11efd7bafb0a5ea9f17f
Bug-Url: https://bugzilla.redhat.com/1109548
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>