add support for 'keypair' argument to launch configs.
add support for 'keypair' argument to launch configs.

EdLeafe/pyrax,mandx/pyrax,rackerlabs/heat-pyrax,ddaeschler/pyrax,vikomall/pyrax,naemono/pyrax,opsdisk/pyrax,0dataloss/pyrax,sivel/pyrax,briancurtin/pyrax,rackspace/pyrax,roelvv/pyrax,pratikmallya/pyrax