Fix puppet on Beluga
After an upgrade to stretch (and with it, an upgrade of puppet from 3.7 to 4.8), puppet won't run anymore:
root@Beluga:/etc/apt/sources.list.d# puppet agent -t
Warning: Setting configtimeout is deprecated.
(at /usr/lib/ruby/vendor_ruby/puppet/settings.rb:1146:in `issue_deprecation_warning')
Info: Not using expired certificate for ca from cache; expired at 2019-04-07 19:04:29 UTC
Info: Not using expired certificate for ca from cache; expired at 2019-04-07 19:04:29 UTC
Error: Could not request certificate: Error 400 on SERVER: The environment must be purely alphanumeric, not 'puppet-ca'
Exiting; failed to retrieve certificate and waitforcert is disabled