feature: preconf

[kazubu]

I would like to add preconf configuration that is originally implemented on slankdev/tinet. I am heavily using it for define global variables used in every node configurations like as below:

preconf:
  - cmds:
      - cmd: ROUTER_ID_PREFIX='10.255.0'

node_configs:
  - name: R1
    cmds:
      - cmd: /usr/lib/frr/frr start
      - cmd: >-
          vtysh -c "conf t"
          -c "bgp router-id $ROUTER_ID_PREFIX.1"
  - name: R2
    cmds:
      - cmd: /usr/lib/frr/frr start
      - cmd: >-
          vtysh -c "conf t"
          -c "bgp router-id $ROUTER_ID_PREFIX.2"

$ tinet conf -c spec.yaml
ROUTER_ID_PREFIX='10.255.0' > /dev/null
docker exec R1 /usr/lib/frr/frr start > /dev/null
docker exec R1 vtysh -c "conf t" -c "bgp router-id $ROUTER_ID_PREFIX.1" > /dev/null
docker exec R2 /usr/lib/frr/frr start > /dev/null
docker exec R2 vtysh -c "conf t" -c "bgp router-id $ROUTER_ID_PREFIX.2" > /dev/null