Sometimes the ephemeral environment needs additional cloud-init data.
This change allows user-data sections to be added to the default files
in /etc/maas/preseeds: enlist, commissioning, and curtin.
For example, to resolve issues with 'apt-get update' failures during
enlistment, something like this may be necessary:
conf:
cloudconfig:
override: true
sections:
bootcmd:
- "rm -fr /var/lib/apt/lists"
Change-Id: I817006a799003ace3f35d02507489720b0f9079b
35fa3175e3