(maint) Add an acceptance pre-suite step to set the parser

Allows us to run acceptance tests with the parser set to the value of
$PARSER from the environment.  We can use this to test how the
acceptance tests fair against the current iteration of the future
parser, for instance.
Merge pull request #2146 from jpartlow/maint/master/acceptance-set-parser

(maint) Add an acceptance pre-suite step to set the parser
Merge branch 'master' of https://github.com/puppetlabs/puppet