Allow for specifying the autoinstall file over the kernel command line

[Chris-Peterson444]

This change allows for specifying the location of the autoinstall.yaml file in the kernel command line via subiquity.autoinstallpath. So, something like subiquity.autoinstallpath=path/to/autoinstall.yaml.

I have additionally changed the precedence for the various ways the autoinstall file may be passed from:

1. root of the drive
2. subiquity cmdline
3. cloud config
4. root of cdrom

To:

1. subiquity cmdline
2. kernel cmdline
3. root of the drive
4. cloud config
5. root of the cdrom

For the tests:

1. I've added a test for the kernel command line case.
2. Reordered them to align with precedence changes
3. Used explicit "missing" values where possible (e.g., None for the subiquity arg when the case is no arg is passed), instead of simply leaving them undefined.

[dbungert]

Doc update please, and please rebase the commits down to one or two. Doc update could arguably be a distinct commit. We don't use the squash merge feature so the commits should be long-term interesting.

Take a peek at recent commit history for the convention being used, for instance "initial fix" is more generic than I would like.

Thanks!

[Chris-Peterson444]

Docs updated and commits rebased. Let me know if you would like the section in the doc to be elaborated further.

[dbungert]

Thanks. One doc tweak and this is done.

[ogayot]

Thanks for move CLI argument up the chain. It feels more natural.