webadmin: Added Import command to Networks main tab

This command is a variation of the network provider Discover dialog,
only here one may choose provider among available network providers
within the dialog. This is more general than Discover, so
DiscoverNetworksModel now inherits from ImportNetworksModel.

Change-Id: I1c84085e19576f6d7de98bccfaf30dc5b70a9621
Signed-off-by: Lior Vernia <lvernia@redhat.com>