Openssh (sshd) doesn't want to start with keys generated by dropbear

slaprunner: updates export script to make sure that etc directory exists in backup before copying into it

When it is restored on importer instance, it erases its RSA key... Which makes the sshd service to crash

resilient: web-takeover.cgi can call a SR-specific python script to display more information on takeover web page

slaprunner: uses sshkeys_authority recipe to manage openssh keys, otherwise they may change at any slapos run

Also makes the interface more readable