azure root-disk constraint fails if < default root disk size

[Kevin W Monroe]

Juju 2.0.1-xenial-amd64 on Xenial:

juju deploy ubuntu --constraints="root-disk=20G"

Deployment fails with a 'provisioning error'. Azure portal gives more details on why it failed:

machine-0 Provisioning failed. Disk size reduction is not supported. Current size is 31457280512 bytes, requested size is 22548578816 bytes.. ResizeDiskError

This happens because the default instance for my given constraint has a 32G root disk. Specifying anything smaller via the root-disk constraint will fail. It would be nice if we know the default root disk sizes for azure instance types. If a root-disk constraint is smaller than that, ignore the constraint.

AFAICT, this does not affect AWS. The disk size for default aws instances is 8G, and even if I specify root-disk=4G, my deployment succeeds. I end up with an 8G root disk, so it seems anything smaller is simply ignored on AWS. I'd like the same thing to happen on Azure.

[Anastasia (anastasia-macmood)]

From IRC conversation with axw, we should add a check for minimum around https://github.com/juju/juju/blob/staging/provider/azure/environ.go#L415.

We currently define one for windows only.