Fix create server request in test_network_v6
Fix create server request in test_network_v6

As Nova V2.1 is much strict about API inputs, Security group name
only is accepted on create server request as a security group
parameter.

test_network_v6 passes complete security group (name, id, rules etc)
in create server request and fails for Nova V2.1.

Closes-Bug: #1414967

Change-Id: I45adde0c7c4134ea087427f23f2e45d7ec9c88e7

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