Remove RPC to plugin when dhcp sets default route

_set_default_route() was using an RPC to the plugin to get the DHCP
port for the network on the current host, and then used it to form
the tap device name. This happened on every allocation reload too.
This fix removes the RPC and gets the tap device name using local
methods instead. It also removes an unnecessary call to set the
default route in the restart method.

Closes-Bug: 1290068
Related-Bug: 1294254
Change-Id: I639bcf93725c4969d1011d2d20491d461ccfdbed
(cherry picked from commit 9dbd1e5e5a41eff88a044b8de992d2f1f14898b3)
Merge "Remove RPC to plugin when dhcp sets default route" into stable/icehouse