Doc: Pacemaker Remote: Explicit firewall step for guest

Give firewalld commands, and include the notes about non-firewalld
configuration and disabling firewalld and SELinux in test environments.

Remove port 5405/udp -- by definition, remote and guest nodes aren't
using this Corosync port.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>