Ubuntu – ubuntu server: wifi

I just installed Ubuntu 14.04 server on my computer that has both a wired network port and a wifi card. I would like for the server to use the wired network if is not available, and use wifi when it is not available.

The original /etc/network/interfaces look like the following:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto p4p1
iface p4p1 inet dhcp

I followed this post and modified my file to be:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto wlan0
iface wlan0 inet dhcp
wpa-ssid <my_ssid>
wpa-psk <my_ssid_password>

# The primary network interface
auto p4p1
iface p4p1 inet dhcp

When I restart wlan0, I'm able to connect to my wifi, no problem. If I restart the computer with my network cable plugged in, no problems either as both the wired and wifi connections connect to my network. However, if I have my wired connection unplugged and reboot, I see the message 'Starting configure network device' at startup. I then see the message 'waiting up to 60 more seconds for network configuration' before the system fully boots (so it waits 120+ seconds for the network). When the computer boots, my wifi connections is connected.

So, I guess things work, but the my headless server takes a long time to fully boot. What am I doing wrong? Is there anyway to fix this bootup lag? Thank you.