gem5: use pre-generated keys instead of haveged in gem5 machine

As the gem5 image won't be used in production there is no need to
generate unique SSH keys on the first boot, which takes a long time in
gem5.

Instead of recommending haveged (from meta-oe) we can simply install
pre-generated SSH keys. These keys are static so should not be used in
production!

Change-Id: I134a421d66eb2b55cf0e4f44781021ac2c96106d
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>