LegacyVersion class is removed from newer setuptools package. But
support for legacy versions is added in python-jenkins 1.8.2.
Switch to that implementation.
Fix broken plugin version comparison for legacy versions.
Assume latest plugin version if no plugin version is found.
Story: 2010990
Story: 2009943
Story: 2009819
Story: 2010842
Task: 49236
Task: 44852
Task: 44396
Task: 48448
Change-Id: Id7f0be1c42357454bd9bedcdee3fefb174943d81