Add SSH class to rally.sshutils

Following methods added:

 * execute: execute command
 * execute_script: execute local script on remote host
 * upload: upload file
 * wait: wait for ssh is available

Added exceptions SSHException and SSHTimeout

blueprint serverdto-ssh

Change-Id: Ibb261e0561e4104f4b41414b53d5a67d8b267a01
Add ssh object to ServerDTO

With this change servers is easy accessible with ssh:

server = ServerDTO(...)
server.ssh.wait()  # wait for ssh is available
server.ssh.execute("reboot")

blueprint serverdto-ssh

Change-Id: I15a962927331cec80b63d759a2717b171ca41e6f