added pssh (https://code.google.com/p/parallel-ssh/) support to knife ssh

If the server provides Kerberos (gssapi-with-mac) only for authentication method, we need another method for executing ssh command in knife, because of ruby's Kerberos implementation(https://github.com/joekhoobyar/net-ssh-kerberos) is outdated. 

So I added pssh support to knife ssh command. 

Here's a sample command usage and output.
```
jaewoo@foo:chef-repo (master*)$ knife ssh 'role:lasvegas_redis' pssh cat /etc/issue
[1] 16:27:24 [SUCCESS] deploy@~~~
CentOS release 6.5 (Final)
Kernel \r on an \m

[2] 16:27:24 [SUCCESS] deploy@~~~
CentOS release 6.5 (Final)
Kernel \r on an \m
```