Add new snippet ubuntu_authorized_keys

Adds a new snippet for setting up the /root/.ssh/authorized_keys in Ubuntu
carriage returns in keys are escaped in the preseed
they are put back with command echo -e
existing authorized_keys renamed centos_*

Change-Id: Ia9b77d418a27f86665671c323ae0e23002907456
Closes-Bug: #1277253
Merge "Add new snippet ubuntu_authorized_keys"