[2.5b3] Unable to deploy server - curtin config SERVER_ROOT

Hi Mark,

Is this something you can constantly reproduce? Were all region/rack controllers connected at the time? Are you accessing MAAS over a VIP, reverse proxy or similar ?

The reason I ask is because it seems like a similar issue we have fixed before [1], which would have prevented all machine deployments. That said, the code should have obtained the SERVER_PORT from the request.

[1]: https://code.launchpad.net/~newell-jensen/maas/+git/maas/+merge/355997

Some relevant logs:

2018-10-17 17:35:44 maasserver: [error] Error while calling ScanNetworks: Unable to get RPC connection for rack controller 'scylla' (8tsb7m).
2018-10-17 17:35:44 maasserver: [error] Error while calling ScanNetworks: Unable to get RPC connection for rack controller 'ypso' (kg8fdt).
2018-10-17 17:35:44 maasserver: [error] Error while calling ScanNetworks: Unable to get RPC connection for rack controller 'charybdis' (fbgtn8).
2018-10-17 17:35:44 maasserver: [error] Error while calling ScanNetworks: Unable to get RPC connection for rack controller 'maas' (4y3h7n).
2018-10-17 17:35:44 maasserver.regiondservices.active_discovery: [info] Active network discovery: Unable to initiate network scanning on any rack controller. Verify that the rack controllers are started and have connected to the region.
2018-10-17 17:35:44 maasserver.rpc.leases: [info] Lease update: commit for 192.168.1.4 on 80:2a:a8:c0:32:f8 at 2018-10-17 17:35:11 (lease time: 600s) (hostname: garage)
2018-10-17 17:35:44 maasserver.rpc.leases: [info] Lease update: commit for 192.168.1.28 on 80:2a:a8:49:cb:89 at 2018-10-17 17:35:11 (lease time: 600s) (hostname: workshop)
2018-10-17 17:35:44 maasserver.rpc.leases: [info] Lease update: commit for 192.168.1.7 on 80:2a:a8:c0:0:fd at 2018-10-17 17:35:17 (lease time: 600s) (hostname: office)
2018-10-17 17:35:44 maasserver.rpc.leases: [info] Lease update: commit for 192.168.1.129 on e0:46:9a:42:37:e9 at 2018-10-17 17:35:29 (lease time: 600s)
2018-10-17 17:35:46 maasserver.websockets.protocol: [critical] Error on request (49) machine.action: Failed to retrieve curtin config: 'SERVER_PORT'
        Traceback (most recent call last):
          File "/usr/lib/python3.6/threading.py", line 864, in run
            self._target(*self._args, **self._kwargs)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 852, in worker
            return target()
          File "/usr/lib/python3/dist-packages/twisted/_threads/_threadworker.py", line 46, in work
            task()
          File "/usr/lib/python3/dist-packages/twisted/_threads/_team.py", line 190, in doWork
            task()
        --- <exception caught here> ---
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 250, in inContext
            result = inContext.theWork()
          File "/usr/lib/python3/dist-packages/twisted/python/threadpool.py", line 266, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 122, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/usr/lib/python3/dist-packages/twisted/python/context.py", line 85, in callWithContext
            return func(*args,**kw)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 885, in callInContext
            return func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/provisioningserver/utils/twisted.py", line 234, in wrapper
            result = func(*args, **kwargs)
          File "/usr/lib/python3/dist-packages/maa...