backend: AuditLogDirector - replace <UNKNOWN> string

<UNKNOWN> is the string for unauthenticated users in the event tab.
Replacing the string to non interactive user.

Change-Id: I81c2cab403ec045a617ed1190df716c1620338a8
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=922520
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>