diskmanager block device list should be sorted to avoid continual "block devices changed"

[Trent Lloyd]

diskmanager detects that the block devices have constantly changed creating not only log spam, but most likely has an impact on database activity and size on the state server.

Most likely this list simply needs to be sorted consistently, see #1345832 for a previous similar issue with bootstrap server APIHostPort

Versions: client, state server and nodes all on xenial with 2.0-beta11-xenial-amd64

root@semisomnolent-evia:/var/log/juju# uptime (freshly installed)
 02:43:46 up 15:19, 1 user, load average: 0.96, 1.28, 1.43

root@semisomnolent-evia:/var/log/juju# grep "block devices changed" machine-1.log |wc -l
1671

Example log entries:
2016-07-10 02:35:58 INFO juju.worker.diskmanager diskmanager.go:62 block devices changed: [{sda [/dev/disk/by-id/wwn-0x55cd2e404be93bc9 /dev/disk/by-dname/sda /dev/disk/by-path/pci-0000:00:1f.2-ata-1 /dev/disk/by-id/ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P] ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P 114473 true }]
2016-07-10 02:38:44 INFO juju.worker.diskmanager diskmanager.go:62 block devices changed: [{sda [/dev/disk/by-id/wwn-0x55cd2e404be93bc9 /dev/disk/by-path/pci-0000:00:1f.2-ata-1 /dev/disk/by-id/ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P /dev/disk/by-dname/sda] ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P 114473 true }]
2016-07-10 02:40:29 INFO juju.worker.diskmanager diskmanager.go:62 block devices changed: [{sda [/dev/disk/by-id/wwn-0x55cd2e404be93bc9 /dev/disk/by-dname/sda /dev/disk/by-path/pci-0000:00:1f.2-ata-1 /dev/disk/by-id/ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P] ata-INTEL_SSDMCEAW120A4_CVDA448501ZJ120P 114473 true }]

[James Tunnicliffe (dooferlad)]

This is due to us sorting on the device name (e.g. sda), but then performing a deep comparison on everything we store, including the device links (by-id, by-path etc), which we don't sort.

Nice easy fix so I will get it queued up.

[Andrew Wilkins (axwalk)]

1.25: https://github.com/juju/juju/pull/7750