bgpd: Fix bgp auto-shutdown behavior

bgpd: Fix bgp auto-shutdown behavior

The current behavior of the `bgp default shutdown` command is to set the
state of all newly configured peers to shutdown. This leads to a problem
when restarting bgpd, because all peers will then be seen as newly
configured, which leads to all peers being set to shutdown after each
restart.

This behavior is undesired and not common when comparing the
implementation against other vendors. This commit moves the `bgp default
shutdown` configuration underneath the peer-group and peer
configuration, to ensure that existing peers will not be set to shutdown
after a daemon restart.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>