Use of ssh-agent when running alde

[rosogon]

When using alde on an untrusted host, ssh keys to connect to testbeds should be passphrased-protected. ALDE can run using ssh-agent as usual:

eval $(ssh-agent)
ssh-add <protected-key>
python app.py

Everything looks working fine except:

2020-07-20 15:57:59,370 root         ERROR    Trying to execute command:  ssh ubuntu@alde ssh alde 'cat /proc/cpuinfo'
2020-07-20 15:57:59,371 root         ERROR    Error: Command ' ssh ubuntu@alde ssh alde 'cat /proc/cpuinfo'' returned non-zero exit status 255.

For fix this, ssh -A to enable forwarding could be used.