neutronv2: only create client once when adding/removing fixed IPs
neutronv2: only create client once when adding/removing fixed IPs

We don't need to create a neutron client object (session) every time we
need to make an API call in add_fixed_ip_to_instance or
remove_fixed_ip_from_instance, especially when looping over ports and
subnets.

Just create the client once at the beginning of the method and reuse it.

Closes-Bug: #1430481

Change-Id: If9181c2ef5fb94652d0205d6d69521bcc5d85cff

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