webadmin: New Host - External provider changes

This patch does the following changes in the new
host dialog, with regards to using external host provider:

1. s/Show External Providers/Use External Providers
2. Hide all fields in case the checkbox is not checked
3. Use the same panel as the DC/Cluster
4. Choose the first host provider automatically when enabling.

Bug-Url: https://bugzilla.redhat.com/979763
Change-Id: I3a4127d47b2670c089eafca34a2800f970269496
Signed-off-by: Oved Ourfali <oourfali@redhat.com>