ui: Changing foreman provider data loading

When selected new provider the addHost external provider form fields will
be cleaned and blocked till all provider's info will be loaded.
In this patch we also change the update method which now retrieves all
provider's info in each call, instead of loading only part of the
information.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1124842
Change-Id: Ie01a6c8b6d6f7e58a6057fc211ab5e4605bbe310
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>