Split system_identity controller from subiquity one
Split system_identity controller from subiquity one

There is little inherited from it, let’s split them.

Co-authored-by: Didier Roche <287ee606043e6024a035a993ea8b6bf47b9638d2@ubuntu.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity