Changed the handling of wls_server 'arguments' property.

It should now be possible to provider server arguments as either a string, or an
array of strings. When an array of strings is supplied, it will be joined by
newlines.
This makes it possible to, when using YAML, use references for repeated
arguments. Example:

server_instances:
  'server1':
    ensure: present
    arguments:
      - &repeatme 'you should repeat me'
      - 'other argument'
  'server2'
    ensure: present
    arguments:
      - *repeatme
      - 'yet another argument