[nailgun] Show notification if disk drive is too small

[Mike Scherbakov]

To avoid hard-to-debug issues like #1317966, which cause provisioning to fail with non-friendly message, let's notify a user that disk drive(s) is too small if so. Requirements for VBox are here: http://docs.mirantis.com/fuel/fuel-4.1/install-guide.html?highlight=virtualbox#virtualbox, and it is 50Gb at the moment.

So there are two ways how we can determine that disk is small:
a) simply using 50Gb hardcoded value, and at the node discovery time we can create notification like "Node ... has too small disk. It may cause deployment issues."
b) using volume manager's logic, determine if for this particular node hardware set it is fine to have disk with such a size or not.

[Mike Scherbakov (mihgen)]

I still think it is Medium, as it affects users - they try Fuel, and fail. The main issue is that they have no clue why it failed. So it creates bad perception of the project.

[Dmitry Pyzhov (dpyzhov)]

Converted to blueprint: https://blueprints.launchpad.net/fuel/+spec/verify-node-config-before-deployemnt