remove all bitbake recipes which use Github private repository

[fsaintma]

• open-a8-autotest image: remove SSH access from IoT-LAB gateway with /home/root/.ssh/authorized_keys creation file
• gateway image: replace IoT-LAB sshd server keys by dummy sshd rsa server key and force openssh installation without read-only filesystem mgt (keys in /etc/ssh instead of /var/run/ssh) remove SSH access from IoT-LAB admin users and servers with /home/root/.ssh/authorized_keys creation file remove IoT-LAB ssh gateway keys copy (/home/root/.ssh/id_rsa and /home/root/.ssh/id_rsa.pub)
• open-a8 image: remove SSH access from IoT-LAB admin users and servers with /home/root/.ssh/authorized_keys creation file