backport floor() func from puppetlabs-stdlib v4.3.2

floor() is included in the 4.x release series. I thought of upgrading us to
that version, but that's a 10,000 line diff filled with garbage we don't need.
So instead I'd like to just cherry-pick the one function that I'd like to use:
floor(). It has no external dependencies.

Obtained from <https://github.com/puppetlabs/puppetlabs-stdlib
  /blob/master/lib/puppet/parser/functions/floor.rb>

Change-Id: I6c68cb465f3d5a09b93efbc663cc85a8863a8deb
Merge "backport floor() func from puppetlabs-stdlib v4.3.2" into production