bgpd: Add 'no set as-path prepend last-as X' command

bgpd: Add 'no set as-path prepend last-as X' command

The `set as-path prepend last-as X` command had no, 'no' form
of the command.  Add this into the cli.

Testing:
!
route-map BLARBLE permit 10
 set as-path prepend last-as 3
!
!
router bgp 9999
 neighbor 10.50.12.118 remote-as external
 neighbor 10.50.12.118 ebgp-multihop 30
 !
 address-family ipv4 unicast
  neighbor 10.50.12.118 route-map BLARBLE in
 !
!

eva# show bgp ipv4 uni 4.4.4.4
BGP routing table entry for 4.4.4.4/32
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  10.50.12.118
  999 999 999 999
    10.50.12.118 from 10.50.12.118 (10.50.12.118)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Last update: Mon Aug 26 09:47:17 2019

eva# conf
eva(config)# route-map BLARBLE permit 10
eva(config-route-map)# no set as-path prepend last-as 3
eva(config-route-map)# end
eva# clear bgp ipv4 uni *
eva# show bgp ipv4 uni 4.4.4.4
BGP routing table entry for 4.4.4.4/32
Paths: (1 available, best #1, table default)
  Advertised to non peer-group peers:
  10.50.12.118
  999
    10.50.12.118 from 10.50.12.118 (10.50.12.118)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Last update: Mon Aug 26 09:48:31 2019

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