Improve import_gpg_keys
    
    - list master keys without 0x in encrypted_pillar_recipients
      -> this is an easy hack to avoid attempted keyserver retrievals
         for keys which are not published on any
    
    - apply ShellCheck suggestions
      - no longer loop over `ls`, use native Bash globbing instead
      - quote variables
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>