Machine unable to pxe with no-such-image while non-related images are being imported

[Andres Rodriguez]

In the CI, I came across a bug that surfaces an issue that prevents machines from PXE booting while other images are being imported. What does the CI do:

1. CI imports Xenial
2. CI tells machines to PXE (power's them on for enlistment).
3. Machine enlists and CI containers.

However, I did this:

1. CI imports Xenial
2. CI tells machines to PXE
3. After 2, I immediately try to import images for other architecture (such as arm64, i386).
4. Machines fail to PXE boot with no-such-image even though amd64 images are there. (while the region is trying to import new images).

Confirmed in 2.0, trunk, bootloader branch.

Example:

#MAAS Imported images in Region:
maas-integration.TestMAASIntegration.test_imported_boot_resources_region ... ok
#MAAS Imported images in Rack:
maas-integration.TestMAASIntegration.test_imported_boot_resources_rack ... ok
#MAAS Told macines to PXE boot
maas-integration.TestMAASIntegration.test_boot_nodes_enlist ... ok

#At this point, I try to import arm64/i386 manually, and this is what I see in the logs while Region is trying to sync new images and machines are attempting to PXE boot:

2016-08-11 08:19:41 [TFTP (UDP)] Datagram received from ('10.245.136.19', 49173): <RRQDatagram(filename=b'ubuntu/amd64/generic/xenial/no-such-image/boot-kernel', mode=b'octet', options=OrderedDict([(b'tsize', b'0'), (b'blksize', b'1408')]))>

[Christian Reis (kiko)]

See also bug 1612203, which appears related.