bgpd: clean up import vrf route-map command

bgpd: clean up import vrf route-map command

Problem seen that if "import vrf route-map RMAP" was entered
without any vrfs being imported, the configuration was displayed
as "route-map vpn import RMAP". Additionally, if "import vrf
route-map" was entered without specifying a route-map name,
the command was accepted and the word "route-map" would be
treated as a vrf name.  This fix resolves both of those issues
and also allows deleting the "import vrf route-map" line without
providing the route-map name.

Ticket: CM-28821
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>