cyclades: Fix removal of stale NICs in BUILD state
Until now, NICs that did not exist in the Ganeti backend and were in BUILD state were removed after a timeout (BUILDING_NIC_TIMEOUT). This was dangerous, because if an OP_INSTANCE_CONNECT job had been "stuck" in Ganeti, the NIC and it's IP address would be released. This commit fixes this by removing such NICs only if the OP_INSTANCE_CONNECT has been finished and the NIC does not exist in the Ganeti backend. Because, querying Ganeti has a time overhead, this is only done for NICs that have not been created too recently.