Fix invalid facilities documented in rootwrap.conf

The values user0 and user1 do not map to valid facility values.
local1, etc.  Using user0 results in a  pri value that does not map
back to a facility of the same name in syslog.

RFC5424 suggest values values of local0 through local7.  Setting
syslog_log_facility to one of those values results in a message with a
priority that can be mapped back to the original string value.

This fix adjusts the comment in rootwrap.conf to suggest the local
prefix instead of the user prefix.

Change-Id: I835ad17c817b8623e382368b39b06944470be480
Closes-bug: #1280879

etc/rootwrap.conf

@@ -17,7 +17,7 @@ exec_dirs=/sbin,/usr/sbin,/bin,/usr/bin
 use_syslog=False
 
 # Which syslog facility to use.
-# Valid values include auth, authpriv, syslog, user0, user1...
+# Valid values include auth, authpriv, syslog, local0, local1...
 # Default value is 'syslog'
 syslog_log_facility=syslog