net/dhcp: bump dhcpcd timeout to 300s (canonical#5127)

On most distros, including Ubuntu, the default timeout for dhclient is 300s.
There is no cloud-init controlled duration for the dhclient process as
it doesn't fork until after it receives an IP address and there is no timeout
value passed to subp().

I have seen some distros configure dhclient with a timeout of 60s, but
is far less common.

Given that a cloud VM is not very useful with DHCP, err on the generous
side and allow up to 300 seconds for dhcpcd to get an address.

Note that there is still an issue with dhcpcd retries which will be
addressed later in a separate PR.

Signed-off-by: Chris Patterson <cpatterson@microsoft.com>

cloudinit/net/dhcp.py

@@ -596,6 +596,7 @@ def get_key_from_latest_lease(self, distro, key: str):
 
 class Dhcpcd(DhcpClient):
     client_name = "dhcpcd"
+    timeout = 300
 
     def dhcp_discovery(
         self,