cri: initial implementation of stage1 changes

[s-urbaniak]

This PR pulls the stage1-based changes from the CRI branch back into
master, leaving out the changes in stage0 (new app subcommands).

Being in the beginning of the next release cycle this gives us the
opportunity to merge the first CRI efforts.

/cc @yifan-gu @lucab @squeed @alban

Relates to #2992

[s-urbaniak]

For a reference here is the stat-diff towards the cri branch:

$ git diff crisync..origin/cri
 Documentation/devel/stage1-implementors-guide.md |  34 +++++++++++++++++++++++++++++++
 rkt/app.go                                       |  28 ++++++++++++++++++++++++++
 rkt/app_add.go                                   | 114 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_exec.go                                  |  37 ++++++++++++++++++++++++++++++++++
 rkt/app_list.go                                  |  65 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_rm.go                                    |  82 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_sandbox.go                               | 199 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_start.go                                 |  98 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_status.go                                | 125 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/app_stop.go                                  |  94 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 rkt/list.go                                      |   1 -
 11 files changed, 876 insertions(+), 1 deletion(-)

[s-urbaniak]

I set the do-not-merge label until I have a green light from CI.

[s-urbaniak]

CI looks green enough for needs/review, PTAL :-)

[lucab]

This unit is still referenced in many places, but it is removed here. How does it work?

[iaguis]

It's generated programmatically in https://github.com/coreos/rkt/pull/3218/files#diff-99ef63dd92c607458ff979e0c391eb56R68

[s-urbaniak]

To add a little bit more context: the halt.target has a Requires/After dependency on shutdown.service in the case of mutable pods. This prevents mutable pods to shut down prematurely if there are no applications started.

[lucab]

LGTM