bgpd: fix import vrf route-map issues

bgpd: fix import vrf route-map issues

Prior to this fix, the import vrf route-map command only worked
if the route-map existed prior to the command.  Additionally, if
the import vrf route-map command was issued without an existing
route-map, the imported prefixes were not removed.  This fix
resolves both of thes mis-behaviors. bgp-smoke run with same
failures as base.

Ticket: CM-20459
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: CCR-7358