[vDNS: VM do not get default DNS IP if IPAM dns method is not set

[Pulkit Tandon]

R4.0 - Build 20

Created a VN and associated a subnet with it.
The subnet to be part of default IPAM

Created a VM.
Observed that VM has no value for name server in resolv.conf

Created a vDNS server and attached it to default IPAM.
Spawned another VM and observed that the VM gets the server IP correctly.

Debugging:
When we do not explicitly set a DNS server method in IPAM, it is set to empty ("").
Somehow, due to some change, it is getting set to "None"

Parsing None is not a part of code. Thus, change is required while setting "IPAM_dns_method" in schema.