De-duplicate networks in provisioning request

In some particular cases end user could provide duplicated networks in
provisioning request by horizion with softlayer network environment, the
change de-duplicate these duplicated network ids and make softlayer
provision the server(s) with proper network(s).

Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
`networks` and `nets` seem to similar here, perhaps `unique_nets` would be more clear.