Change the way Ansible lookups SSH public keys

Current popular lookup which accesses '~/.ssh/id_rsa.pub' file directly
is not portable. Because of that, DebOps playbooks and roles will move
to read the list of public SSH keys directly from SSH agent using
'ssh-add -L' command.
Merge pull request #14 from drybjed/change-id_rsa-lookup

Change the way Ansible lookups SSH public keys