remove unneeded call to network_api on rebuild_instance

When rebuilding an instance we call the network_api and which results in
calling neutron and updating the info cache. We do not actually need to
do this as we can get the nw_info directly from the instance's info cache.

Change-Id: I2bcc2f15b1e5bb52d972098c7bc3e12f160b6feb
Closes-bug: 1294914
Merge "remove unneeded call to network_api on rebuild_instance"