Fix examples/platform-aws.yml: add dhcpcd service to keep instance reachable after lease

[jf]

- What I did

add dhcpcd as a service as well in order to have the instance not invariably become unreachable after the lease expires

- How to verify it

1. Follow the instructions in the docs to create an AMI
2. create an instance from the AMI
3. wait 3600 seconds (or however long the lease is in your case)
4. observe how the instance loses connectivity at the very second after the lease expires
5. now create another AMI, but with this fix and observe how the instance stays up (I have one instance that hasn't become unreachable, with an up time of over 22 hours)