add(ssh_config): Move ssh configs from coreos-base to init.

Going forward the coreos-init package will cover configuring and
enabling system daemons. Previously this was shared with coreos-base.
fix(sshd_keygen): Split host key generation into its own unit.

This new service will always during boot run before sshd, both socket
and daemon modes are supported.
fix(coreos-init): Finish splitting coreos_startup, properly mount /var

Handle /home and /var with mount units and use systemd's built in tmp
file recreation scheme for creating missing directories in the state
partition. This trims the old script down to just mounting /usr/local in
dev_mode which is fine to leave as-is.

There is a new coreos-startup.target unit which simply depends on every
other unit we want so we don't need add things individually in the
coreos-init ebuild.

Since /var is now backed by /mnt/stateful_partition and tmpfs booting
with a read-only root filesystem might even work now. :)
fix(tmpfiles): remove conflicting definition for /var/log

The default permissions are root:root and nothing seems to be using the
syslog group so I assume it is not relevant on our systemd based setup.
Merge pull request #3 from marineam/boot-cleanup

Boot cleanup