Java gen: fix alignment for action set_field, add java_gen unit tests for actions

This is a resubmit of PR https://github.com/floodlight/loxigen/pull/101 ( I messed up the merge) + add a fix for internally aligned messages

Reviewer: @rlane @vaterlaus @yotamhc

This pull request fixes the alignment of the class OFActionSetField. Actions must be aligned to 64 bits according to the spec, and the alignment is included in the length (vs. the MatchV3 class, where the alignment is not included in the length sigh).

This PR adds support for declaring whether the length includes the alignment or not to the input files (param length_includes_align). It also changes the unit test lookup mechanism somewhat to make java_gen find the set_action-related unit tests.