Rename ssh identity file option

In order to match the options from knife bootstrap and avoid
option namespace problems.  The old --ssh-identity option is
kept but deprecated in favor of the new --identity-file option.
If both options are specified the new option takes precedence
over the older deprecated option.
Deprecation message for `--ssh-identity` tells user the new option

Also added ui warning message if `--ssh-identity` is used.
Rename validate_first_cli_arg_is_a_hostname to validate_ssh_options
Move handling of the --ssh-identity to #validate_ssh_options!
Merge branch 'rename_ssh_option'

Fixes #178. Thanks to @russellcardullo.