Check that the kernel parameters meet the LXD production setup reference values

The LXD project provides a series of reference values for kernel parameters for production clusters [0]

One of them for example is `kernel.keys.maxkeys`, which by default is 200, but has a recommended value for production clusters of 2000.

Not meeting this number causes issues like LP#1891223 [1], which can trigger during a deployment or worse during an upgrade on a metal with many containers and break the upgrade itself.

It would be helpful to have a check for this condition, at least for the kernel.keys.* settings.

[0] https://linuxcontainers.org/lxd/docs/master/production-setup/
[1] https://bugs.launchpad.net/juju/+bug/1891223