support for proxy_command parameter

[chidanandpujar]

prox_command parameter support

regress@masterhost:~/junos_eznc_proxy$ cat test.py 
from jnpr.junos import Device

dev = Device(
    host="xx.xx.xx.xx",
    user="user1",
    port=22,
    ssh_private_key_file="~/.ssh/id_rsa",
    proxy_command="ssh -W %h:%p -q user1@xx.xx.xx.xx",
)
dev.open()
print(dev.facts)
dev.close()