fix(sdk_lib): Test for host file existence before copying.

The main case here is /etc/hosts does not exist on CoreOS. In the
process combine related and duplicate code. Setting the timezone now
happens in entire_chroot like hosts and resolv.conf. Don't bother with
setting a default UTC time zone, that is already the default.
fix(sdk_lib): Clean up ssh file copying

- Don't copy known_hosts if it doesn't exist.
- Don't bother with copying *.pub, not sure what that was for.
- Don't rewrite .ssh/config to remove internal Google ssh options.
fix(sdk-lib): Mount a fresh /run directory.

There is no need to arbitrarily bind mount all of the host system's /run
into the chroot. In fact this causes issues when the host system's /run
isn't set up in a way this script anticipates. Namely the user runtime
directory in /run/usr/$UID is another tmpfs mount on my system, leaving
the underlying directory node that is bind-mounted in with the wrong
ownership. Behave a little more like a responsible container and use a
fresh /run but continue binding /run/shm for whatever versions of Ubuntu
that depended on that behavior. Not strictly needed but go ahead and
create the user runtime directory with the correct permissions.
Merge pull request #261 from marineam/coreception

Fixes for SDK setup