fix location sanitizing with parser 'future'

With parser 'future', a '\\' counts as one backslash whereas with
parser 'current' it's two. Therefore with 'future' only one gets passed
to regsubst which in turn aborts with

 Error: regsubst(): Bad regular expression `\'

The 'future' behaviour is indeed the correct one but 'current' won't be
fixed either. Therefore, use "\\\\" which produces two backslashes with
both parser (ref: https://tickets.puppetlabs.com/browse/PUP-1814).