(IAC-889) Set FastGettext.default_text_domain in Rakefile

[sanfrancrisko]

The spec tests were failing due to the metadata_lint Rake task exiting with a return code of 1:

bundle exec rake check:symlinks check:git_ignore check:dot_underscore check:test_file rubocop syntax lint metadata_lint; echo $?
Running RuboCop...
Inspecting 0 files


0 files inspected, no offenses detected
---> syntax:manifests
Current textdomain ("master_domain") was not added, use FastGettext.add_text_domain !
1

See this run on TravisCI too.

According to MODULES-6598 there is a conflict with GetFastText initialisation between puppet_semantic and puppet.

This PR implements the workaround described in MODULES-6598](https://tickets.puppetlabs.com/browse/MODULES-6598), which is to set the default_text_domain parameter in GetFastText