Refactor the SSH field in v1alpha2

[luxas]

• Remove the custom marshaller/unmarshaller, for more ease of use
• Add the option to specify key algorithm (Changing the ssh key gen algorithm for FIPS machines #136)
• Add validation

[luxas]

@twelho wrote in #152:

In FIPS-enabled machines the ed25519 key generation algorithm is unavailable, so they need to use rsa.

Let the user specify the algorithm to use via a flag during VM creation
Improve autodetection for FIPS machines to automatically switch over to rsa
Enforce a specific key length (between 2048 and 4096) when using rsa

We'll most likely fix this in v1alpha2, but I'm not certain we have the time.
I'm not sure if we should keep the auto-detection of the SSH key algorithm, or just "blindly" use what we got from the API. An other alternative would be to try to generate an ed25519 key and if that fails, fallback on rsa.
Anyways, this code needs some love.

[luxas]

After thinking about this, it's not high-priority enough to do in v1alpha2 / v0.5.0. Moving to v1alpha3.