rhel: Remove the firewall hole that we create for GRE.

rhel: Remove the firewall hole that we create for GRE.

Till now, by default, we add firewall holes for
gre traffic. There may be users that do not use GRE tunnels
and they may be surprised with this behavior. So, don't add
the firewall rules by default and update the documentation
to mention the same.

This patch does not remove the default GRE firewall rule for
xenserver because xenserver has a feature called "Cross-Host
Internal Networks" (CHIN) that uses GRE.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>