Reduce cyclomatic complexity of ReconcileLoadBalancer

This function had become genuinely too complex over time, to the point
that even the linter was starting to complain about it when making
almost any change.

This change refactors ReconcileLoadBalancer into several smaller logical
functions which are much easier to read and reason about. It also
revealed some trivial optimisations:
* Only fetch Octavia providers if we need them to create a new
  loadbalancer
* Only calculate allowed CIDRs once
* Don't re-fetch a loadbalancer to check it's active if it's already
  active

Author: mdbooth