On initial boot when networking is brought up by cloud-init this
is the timeout that dhclient adheres to. Centos configures
"timeout 300" (for an EC2 bug) in their cloud image, which results
in a 5 minutes delay to boot in cases where no dhcp available (e.g. IPv6
SLAAC). To reduce this boot delay and to provide consistency with
places where we have set other dhcp timeouts set this to DIB_DHCP_TIMEOUT.
Change-Id: I119a002070501c3dfe7c6730b07ee25f422b85b0
Related-Bug: #1758324