Fix --re=bob to match bob@example.com when using HTTP_LDAP

When we introduced HTTP_LDAP support we broke expansion of the local
username in situations like the --reviewer flag during receive.  This
was because the EmailExpander wasn't configured, as we usually rely on
the LDAP directory to return the address for us.

Bug: GERRIT-263
Change-Id: I8c8d5ce9e5ac1cd1a8b1dc85422257b00e1114ab
Signed-off-by: Shawn O. Pearce <sop@google.com>