Remove spaces from SSH public key comment

An SSH key generated by Nova contains the comment 'Generated by Nova'. Older
versions (prior to 0.7.2) of cloud-init trip over the spaces in the comment
and as a result of that the key injected into the root account is not disabled
by cloud-init. Yes, it's a cloud-init bug but it's also a regression in the
sense that older OpenStack installations (Essex and older) don't contain
spaces in Nova generated key comments and thus older cloud-init's are not
affected in these environments.

This patch replaces the spaces with dashes, i.e., 'Generated-by-Nova'.

Spaces were introduced by commit: 114109dbf4094ae6b6333d41c84bebf6f85c4e48
Ubuntu cloud images with cloud-init < 0.7.2: 10.04 LTS and 12.04 LTS
cloud-init bug report: https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/1220273

Closes-Bug: #1297685
Change-Id: I1761f61dfbba58be98351ae4a51884b03268cf09
Merge "Remove spaces from SSH public key comment"