Fix various pylint violations

Fix various pylint violations

+ SchedUtils.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ VatHistory.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ VppCounters.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ Memif.py:
++ Do not use `len(SEQUENCE)` to determine if a sequence is empty
++ Either all return statements in a function should return an expression,
   or none of them should.
++ Update :return: on possible None.
+ Classify.py: Unnecessary "else" after "return"
+ ContainerUtils.py: Useless super delegation in method '__init__'
+ CpuUtils.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ DropRateSearch.py: Either all return statements in a function
  should return an expression, or none of them should.
+ IPv4NodeAddress.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
++ Also improve docstrings.
+ IPv4Setup.py: Useless super delegation in method '__init__'
+ IPv6Setup.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
++ Also improve docstrings.
+ IPv6Setup.py: standard import "from ipaddress import IPv6Network"
  should be placed before "from robot.api import logger"
+ MacSwap.py: Trailing newlines
+ NATUtil.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ NodePath.py: Unnecessary "else" after "return"
+ Tap.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ topology.py: Either all return statements in a function
  should return an expression, or none of them should.
+ topology.py: Unnecessary "else" after "return"
++ Do not use `len(SEQUENCE)` to determine if a sequence is empty
++ Improve docstrings
+ DUTSetup.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
++ Also do not compare int(ret_code) just to access zero-ness.
+ ssh.py: Do not use `len(SEQUENCE)` to determine if a sequence is empty
+ InterfaceUtil.py: Unnecessary "else" after "return"

Change-Id: Iba4244aa79661ee7df15fed5c7c6dbf04dfa88b2
Signed-off-by: Vratko Polak <vrpolak@cisco.com>