impex2dot: support multiple servers and attract traffic
impex2dot: support multiple servers and attract traffic

bagpipe-impex2dot now supports --server multiple times
to retrieve info from multiple servers and build one graph.

For instance:
bagpipe-impex2dot --server lnx01 --server lnx02 > graph.dot

It also now supports the display of flow spec routes for
attract traffic.

openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2,openstack/networking-bagpipe-l2,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2