Fix pylint unbalanced-tuple-unpacking warning

Fix pylint unbalanced-tuple-unpacking warning

Pylint last version(1.4.1), at least, reports an
unbalanced-tuple-unpacking warning[1] in keepalived[2] module because
self.authentication is defined as an empty tuple in __init__ method and
unpacked in build_config method as if it was a 2-tuple.

self.authentication references an empty tuple (defined in __init__
method) or a 2-tuple (updated in set_authentication method). Such
warning is a false positive because the unpacking is only performed if
self.authentication is not evaluated to false which only appends if
self.authentication is a 2-tuple.

Defining self.authentication as None in __init__ avoids such warning
without disabling unbalanced-tuple-unpacking warning check.

[1]
W:252,12: Possible unbalanced tuple unpacking with sequence defined at
line 153: left side has 2 label(s), right side has 0 value(s)
(unbalanced-tuple-unpacking)

[2] neutron.agent.linux.keepalived

Change-Id: Ifcdf08e574ef44a65c6d121323cbe31d9af2f921
Closes-Bug: #1411865