Refactor code in update_subnet, splitting into individual methods

The update_subnet was already long and complex enough and adding
more code to it was undesirable, after commit

I47a3a71d0d196b76eda46b1d960193fb60417ba9

Hence this refactor splits parts of the update_subnet code into
dedicated sub-methods.

Change-Id: I8f98938da88db287290493dc0daf7c81b8b5f542
Merge "Refactor code in update_subnet, splitting into individual methods"