Force PEM filetype with ssh-keygen
Force PEM filetype with ssh-keygen

At some point ssh-keygen changed its default output format and now
python cryptography can't read the new default. Address this by forcing
the PEM output format which cryptography can read without problems.

Note this is a docs only change to help end users generate keys that
work with refstack after a user reported trouble with this.

Change-Id: I9d1f503817fee69fe22f19a98b506448bda1f888

openstack/refstack,stackforge/refstack,stackforge/refstack,openstack/refstack,stackforge/refstack,openstack/refstack,openstack/refstack,stackforge/refstack