Original validator checked for domain labels as defined by RFC1034, however real
internet deals with other domains as well - starting with digits or symbols.
This change allows modifying the pattern to allow custom / relaxed rules.
Validation has been removed from adding a domain to a new extension, since it's
only used in fixups and the domain should be already validated. (or not, if not
configured)
Closes-bug: 1592489
Change-Id: Ib453054ba5f554bab28cff392c539e713fa28918
2257b8bbed