SDK: Improve the instance creation

Use the nova_client.networks manager to find a network. The sample uses
a 'private' network as created by devstack.

Also rename the 'nic_d' variable 'nics' since it's a list and not a
dict.

Change-Id: I85e53497613421b4bcf604d6033ea31a1be1d020
Merge "SDK: Improve the instance creation"