webadmin: Moved provider list initialization to NetworkModel

This to populate the list box correctly in all occurences of the
dialog (not just in networks main tab).

Change-Id: I0856a02c14eafe1fe9b1867df1a3e9ef4d7e20d4
Bug-Url: https://bugzilla.redhat.com/989898
Signed-off-by: Lior Vernia <lvernia@redhat.com>