mounts: support reliably detecting and using Azure ephemeral disks

Azure's ephemeral disks are not guaranteed to be assigned the same name by
the kernel every boot. This causes problems on ~2% of Azure instances, and
can be fixed by using udev rules to give us a deterministic path to mount;
this patch introduces those udev rules and modifies the Azure data source
to use them.

Changes to a couple of config modules were also required. In some places,
they just needed to learn to dereference symlinks. In cc_mounts this
wasn't sufficient because the dereferenced device would have been put in
/etc/fstab (rather defeating the point of using the udev rules in the
first place). A fairly hefty refactor was required to separate "is this a
valid block device?" from "what shall I put in fstab?".

LP: #1411582

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