Implement SSH keepalives, default 30-second interval, for NETCONF over SSH sessions.

[stacywsmith]

Without SSH keepalives, a NAT or stateful firewall along the network path between the PyEZ host and the target Junos device, may timeout an inactive TCP flow and cause the NETCONF over SSH session to hang. Sending SSH keepalives avoids this situation. The default value is 30 seconds. Setting this parameter to a value of 0 disables SSH keepalives.

Note: This is a different situation than Issue #663 in which the target Junos device is timing out the NETCONF over SSH session due to a configured idle-timeout on the system login class.

[spidercensus]

There should be a check to make sure that this value in the ssh config file is not overridden.

[vnitinv]

Can one of the admins verify this patch?