Convert webdav etc group to use render_ctx

PR: https://github.com/truenas/middleware/pull/8444

Apart from having shared context (which is probably of negligable
benefit here). This also makes several changes to how webdav
config files are generated.

We now always ensure that password hashes are owned by webdav user and
permissions are 0o600 and use secrets.choice() rather than
random.choice() for generating hashes. A subprocess call to change
owner has also been replaced with shutil.chown(). Futher investigation
may be required in order to determine whether this chown call is
actually needed.