Refactor nova.virt.ironic.driver get_host_stats

Refactor the get_host_stats() method to be cleaner
and easier to maintain.

This patch does not change any APIs or functionality.

The unit test for get_host_stats() no longer made sense,
so it was removed.

Co-Authored-By: David Shrewsbury <shrewsbury.dave@gmail.com>

Change-Id: I1b49accfef546d87044f65dcb42b9bef8ab05c6d
Merge "Refactor nova.virt.ironic.driver get_host_stats"