[BZ 1015334] EAP 6 host controller domain host update results in managed servers no longer being "manageable"

This commit deprecates 'domainHost' read-only pluginConfiguration property.
Instead new trait called 'Domain Host Name' was introduced.
BaseServerComponent#validateServerAttributes() now also checks for HC name
and if needed re-reads it from host.xml file. When <host name= changes,
deprecated 'domainHost' property does not get updated (since there is no way
from plugin avail code to update pluginConfiguration on server).
BaseServerComponent now handles 'domainHost' in memory, this variable gets
updated on component start or in avail code (when server was DOWN and now is
UP). Managed servers have domain host name as part of resource key, they get
correctly discovered when this change occurs.