flow: take vlan_id's into account in the flow hash

In VLAN we can have 2 layers of encapsulation. In this patch both
layers are used in the flow hash to distinguish between encapsulated
traffic.
Add yaml option to disable vlan ids hashing

In some cases using the vlan id(s) in flow hashing is problematic. Cases
of broken routers have been reported. So this option allows for disabling
the use of vlan id(s) while calculating the flow hash, and in the future
other hashes.

Vlan tracking for flow is enabled by default.
vlan: add rule for new 'too many layers' event