Add sanity/functional test of net.conf in pre-flight check

[bpwook]

It can be nicer if ACK test IP can be obtained properly on both primary and vlan network during pre-flight check. This can save large amount of time re-running vlan tests without knowing root-cause.

[bpwook]

It is also worth checking all ethX have corresponding dummy bridge, xenbrX.

[bpwook]

• We also need to check that vlan is on different subnet from primary when they uses static IP.