Fix #1554: Improve compare_config for NXOS partial merging

[cmsirbu]

NXOS partial merges were not aware of command hierarchy when showing the differences to be applied via the compare_config method. We (myself and @mundruid) have changed the merge diffing to use the netutils package. Docstrings and documentation updated as well.

Tests would be a stretch goal for another day since this code path is not tested at the moment.

See an example below of old and new behavior:

Intended configuration:

candidate_cfg = """
interface loopback0
  ip address 10.1.4.5/32
  ip router ospf 100 area 0.0.0.1
"""

Base - full configuration on device with the relevant section below:
...
interface loopback0
  ip address 10.1.4.4/32
  ip router ospf 100 area 0.0.0.1
...

nxos1.load_merge_candidate(config=candidate_cfg)

Before:

print(nxos1.compare_config())
  ip address 10.1.4.5/32


With netutils, respecting required command hierarchy:

print(nxos1.compare_config())
interface loopback0
  ip address 10.1.4.5/32

Fixes #1554