fix recursive reauth failure

If both password and token are specified, token is always used in Identity API v2.
Even after token is expired, the expired token is used for re-authentication. This
fixes recursive re-authetication failure by clearing token before calling reauth method.

# Please enter the commit message for your changes. Lines starting
# with '#' will be kept; you may remove them yourself if you want to.
# An empty message aborts the commit.
# On branch fix_reauth
# Your branch is up-to-date with 'origin/fix_reauth'.
#
# Changes to be committed:
#	modified:   openstack/client.go
#
diff --git a/openstack/client.go b/openstack/client.go
index 6818d9d..aaf940b 100644
--- a/openstack/client.go
+++ b/openstack/client.go
@@ -109,6 +109,7 @@
 
 	if options.AllowReauth {
 		client.ReauthFunc = func() error {
+			client.TokenID = ""
 			return AuthenticateV2(client, options)
 		}
 	}