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

1 files changed, 1 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index d4ad6caf..661e968b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -55,6 +55,7 @@
  - ssh: generate ed25519 host keys (LP: #1461242)
  - distro mirrors: provide datasource to mirror selection code to support
    GCE regional mirrors. (LP: #1470890)
+ - add udev rules that identify ephemeral device on Azure (LP: #1411582)
 0.7.6:
  - open 0.7.6
  - Enable vendordata on CloudSigma datasource (LP: #1303986)