feature: support real-time

[John Lettman]

Currently, Nova supports configuring real-time instance policies [0]. This feature can be advantageous for application performance in several cloud environments- particularly telecommunications. The underlying automation of charms is where this feature implementation could shine, handling the multiple moving parts to enable real-time.

Managing the installation of the real-time kernel is likely out-of-scope for this charm and could introduce complexities to the machine lifecycle. It makes sense to leave it with MAAS since it supports real-time kernels via cloud-init modifications to enable Ubuntu Pro [1]. The charm could set a `blocked` state when it fails to detect the real-time kernel on a unit configured to run this feature. The detection component could draw from several system data sources, such as the kernel build configuration.

Likewise, a charm option to flip the feature on would be an excellent addition.

[0]: https://docs.openstack.org/nova/zed/admin/real-time.html
[1]: https://maas.io/docs/how-to-deploy-a-rt-kernel-via-cloud-init