webadmin: Added network provider to Install Host

Refactored code that would be common to Add Host and Install Host into
HostNetworkProviderModel on the model side and
HostNetworkProviderWidget on the view side, and then plugged it to
Install Host.

Change-Id: I3748b7c8b7f7cd277bf1929a8a91168793501696
Signed-off-by: Lior Vernia <lvernia@redhat.com>