fix ssh retcode evaluation when no script is used
add output grabbing execute

This is usefull here to rewrite the keyscan method
refactor keyscan and ssh, add retry on failures

Try to workaround ssh restarting during vm creation.
This should help the initscript upload/keyscan fail less often.

Improve this part until --script is uploaded to Gandi for a swapdisk
bootstrap.
replace check_call with Popen().communicate()

This avoid breaking CLI compat with python < 2.7
Merge pull request #59 from kalou/ssh-fixes

Various ssh fixes