zebra: Use ROUTE_ENTRY_INSTALLED as decision for route is installed

zebra: Use ROUTE_ENTRY_INSTALLED as decision for route is installed

zebra is using NEXTHOP_FLAG_FIB as the basis of whether or not
a route_entry is installed.  This is problematic in that we plan
to separate out nexthop handling from route installation.  So modify
the code to keep track of whether or not a route_entry is installed/failed.

This basically means that every place we set/unset NEXTHOP_FLAG_FIB, we
actually also set/unset ROUTE_ENTRY_INSTALLED on the route_entry.
Additionally where we check for route installed via NEXTHOP_FLAG_FIB
switch over to checking if the route think's it is installed.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>