Fix bug during handling multiple iface sections for same interface

Fix bug during handling multiple iface sections for same interface

Ticket: CM-1438
Reviewed By:
Testing Done: Tested ifupdown2 sanity + multiple iface sections for an
interface

- This patch fixes a few shortcomings in the multiple iface sections
for same interface (partly because i was only covering
backward compatibility cases earlier)
- Since this is a very common configuration pattern, this patch cleans
  it up
- also restructures some code
- main change is:
    before:
        for iface in ifaces:
            for op in ops:
                run op on iface

    after:
        for op in ops:
            for iface in ifaces:
                run op on iface