RESTenv: fork worker: fallback to 'root@pam' for task log user-id

RESTenv: fork worker: fallback to 'root@pam' for task log user-id

The fallback had a "typo" in the realm and used 'root@pve' (pve vs.
pam) since initial import from SVN, but off-list talks with Dietmar
suggest that 'root@pam' was always the intended fallback value.

Call sites without a defined user parameter (found only push_file and
pull_file from pve-container) were logging the task-owner user as
'root@pve' which isn't a default one, so it wouldn't exist in most
setups.

For clarity, add a comment that this is only used for the task logs.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>
[ T: Reword/add to commit message slightly ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>