nf_conntract update

The recipe was using a strict integer check, this made it impossible for the
resource provider to use as the resource provider only accepts string input.

- Add nf_conntrack_max to base

(cherry picked from commit efccacd3b3575f067bf874800d6a6c7c3cb7ae2a)
(cherry picked from commit 5e96cc66a4490b27a98fe5824867280afb72c6ff)

See rcbops/chef-cookbooks#817