Add paramiko ssh agent forwarding (optional)

Hello,

After our discussion on the ML, this patch improves the paramiko ssh transport in order to allow for agent forwarding.

option name : paramiko_connection.agent_forwarding
env option name : ANSIBLE_PARAMIKO_AGENT_FORWARDING

This option can take 4 progressive values :
 * none (default): no agent forwarding
 * user : allow agent forwarding for the ansible ssh user
 * sudo : allow agent forwarding for the ansible ssh user & root via sudo or sudo_user
 * sudo_user : allow agent forwarding for the ansible ssh user & root & any sudo_user

please note that for security reasons, the sudo_user != root case is only available when setfacl is available on the remote host

this implementation raise explicit errors when:
 * paramiko version does not support agent forwarding
 * setfacl is not present on the remote host

I hope this helps