Allow user to specify alternative ssh usernames (ssh_alternate_usernames kwarg)
which are used with deploy_node if the default one doesn't work.

Contributed by Chris Psaltis, part of LIBCLOUD-309.


git-svn-id: https://svn.apache.org/repos/asf/libcloud/trunk@1460106 13f79535-47bb-0310-9956-ffa450edef68