chore(sys-apps/systemd): Roll back to systemd 212

Versions 213 and 214 are not currently usable on GCE:

 - networkd aborts with "Assertion 'link->manager->rtnl' failed"
   https://bugs.freedesktop.org/show_bug.cgi?id=79789

 - persistent naming for virtio devices was removed,
   network devices have returned to the old eth0, eth1, etc. names:
   https://github.com/systemd/systemd/commit/bf81e792f3c0aed54edf004c1c95cc6f6d81d0ee
   This re-breaks our copy of the GCE address_manager agent:
   https://github.com/coreos/coreos-overlay/blob/a24dfc97a91262de07c3da127388c23faaab334d/app-emulation/google-compute-daemon/files/0001-hack-address_manager-use-CoreOS-names-and-locations.patch

This reverts commit da3226051f and f179df3449.
fix(sys-apps/systemd): Add resolv.conf, remove gentoo-integration

This backports commits 18e58b3e9b and 8577b6480b1 from 213 to 212.
chore(sys-apps/systemd): Sync live ebuild with upstream changes.

Only important thing here is that xattr support is now enabled by
default and the libattr dependency is gone. Previously CoreOS didn't
have the feature enabled.
bump(sys-apps/systemd): Add version 214 for testing

Features the same issues that currently block using 213 but here for
testing purposes.
Merge pull request #686 from marineam/systemd

Revert back to systemd 212