Detect and handle existing links
Detect and handle existing links

When adding a new link, it is possible that a link by that name already
exists and may be up. In the case that the link is up the process should
stop. In the case that the link is down, it should be removed and re-added.

Signed-off-by: James Guthrie <474ba67bdb289c6263b36dfd8a7bed6c85b04943@prodigi.ch>

mrvicadai/pipework,indykish/pipework,haggaie/pipework,PeerXu/pipework,palfrey/pipework,hayderimran7/pipework,abn/pipework,SivagnanamCiena/pipework,osrg/pipework,ONode/pipework,snowygoose/pipework,sharry007/pipework,deitch/pipework,zhouqingyu/pipework,wrenth04/pipework,bing6/pipework,YujiOshima/pipework,jpetazzo/pipework,mirzawaqasahmed/pipework,JamesGuthrie/pipework,awanke/pipework,midonet/pipework,OnePaaS/pipework,spheresh/pipework