xenapi: set dhcp timeout on VM installation

Set the DHCP timeout to 120 seconds during virtual machine installation.
Some users failed to run devstack, due to a low DHCP timeout setting.
The default value is 60 seconds. This change sets the value to 120 secs,
that should give enough time for most people.

Change-Id: I15fde45ed0d005c1a8621134eee6c3c338b5be5d
Merge "xenapi: set dhcp timeout on VM installation"