Simplify syntax for default mounts, read-only home + read-write tmp

[afbjorklund]

Description

Currently this block is duplicated to most of the templates:

mounts:
- location: "~"
- location: "/tmp/lima"
  writable: true

Including a hard-coded (global) path to the temporary dir...

• #2339

Simplified syntax, expanded to the above when loaded:

mounts:
- default