fakevm-agent-ryu: add check tunnel device type

In the multi-node-emulation mode, restricts type of target device which
is given a tunnel IP address at initialize or is removed at clean.
fakevm: ensure bridge mode of multi node emulation

fakevm_agent_plugin_base.py:
  - _bridge_exists
    removed this function, because it does not work as expected.
  - _ensure_bridge
    use the bridge name which is specified at a parameter.

fakevm_agent_ryu:
  - OPTS.use_tunnel
    fix the option type to a boolean.
  - _init_bridge
    create a veth device and connect the integration_bridge and the
    vir_bridge by the veth.