ensure Puppet::Util::Execution.execpipe always run the command with LANG...

...="C" and LC_ALL="C" to avoid localized output

This a more generic fix than the one made here: https://github.com/puppetlabs/puppet/pull/2198
With this fix, no more annoying message on french environment (and other non-english machines)

It solves the following ticket: https://tickets.puppetlabs.com/browse/PUP-1401
Sorry for not knowing how to link JIRA ticket and github PR.

all rspec tests are running OK on my machine.