"OS ubuntu with release trusty has no available images for download" when configuring boot-sources

maas-deployer now has an ability to reconfigure maas boot-sources with http://bazaar.launchpad.net/~maas-deployers/maas-deployer/next/revision/68. However it has a race condition when destroying and reconfiguring boot-sources and boot-source-selections. maas-deployer needs to sleep a little bit before configuring boot-source-selections or properly wait until simplesteam index update finishes on MAAS side.

How to reproduce:

$ cat <<"EOF" | tee reproduce-test.sh

set -e

for i in {1..10}; do
    old_source=$(maas ubuntu boot-sources read | jq -r '.[].id')
    maas ubuntu boot-source delete "$old_source"

    maas ubuntu boot-sources create \
        url='http://maas.ubuntu.com/images/ephemeral-v2/releases/' \
        keyring_filename='/usr/share/keyrings/ubuntu-cloudimage-keyring.gpg'

    new_source=$(maas ubuntu boot-sources read | jq -r '.[].id')

    ### needs some sleep here
    maas ubuntu boot-source-selections create "$new_source" \
        release='trusty' arches='amd64' labels='release' os='ubuntu' subarches='*'
done

EOF

$ bash reproduce-test.sh
Success.
Machine-readable output follows:

Success.
Machine-readable output follows:
{
    "url": "http://maas.ubuntu.com/images/ephemeral-v2/releases/",
    "keyring_data": "",
    "keyring_filename": "/usr/share/keyrings/ubuntu-cloudimage-keyring.gpg",
    "id": 30,
    "resource_uri": "/MAAS/api/1.0/boot-sources/30/"
}
{"os": ["OS ubuntu with release trusty has no available images for download"]}