bgpd: Set NO_ADVERTISE community if blackhole community received

rfc7999:
A BGP speaker receiving an announcement tagged with the BLACKHOLE
community SHOULD add the NO_ADVERTISE or NO_EXPORT community as
defined in [RFC1997], or a similar community, to prevent propagation
of the prefix outside the local AS.  The community to prevent
propagation SHOULD be chosen according to the operator's routing
policy.

Sent:
```
router bgp 65534
 no bgp ebgp-requires-policy
 neighbor 192.168.0.2 remote-as 65030
 !
 address-family ipv4 unicast
  redistribute connected
  neighbor 192.168.0.2 route-map spine out
 exit-address-family
 !
!
ip prefix-list self seq 5 permit 192.168.100.1/32
!
route-map spine permit 10
 match ip address prefix-list self
 set community blackhole
!
```

Received:
```
spine1-debian-9# show ip bgp 192.168.100.1/32
BGP routing table entry for 192.168.100.1/32
Paths: (1 available, best #1, table default, inform peer to blackhole prefix)
  Not advertised to any peer
  65534
    192.168.0.1 from 192.168.0.1 (192.168.100.1)
      Origin incomplete, metric 0, valid, external, best (First path received)
      Community: blackhole no-advertise
      Last update: Thu Jan 21 12:56:39 2021
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>

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