Add generic /cmd entrypoint w/ cli support

[alexlarsson]

If you do

   docker devmapper:resize -o Key=Value -o Foo=Bar plugin:op arg1 arg2

This will do a GET http request to /cmd/devmapper/resize with the arguments and options as keys. This will then try to spawn a job on the engine called "devmapper:resize" with the arg1, arg2, etc as arguments and the -o options as environment.

Also, any graph driver implementing the engine.Installer interface will get installed on the engine when the daemon is created.

This will be used by e.g. the devicemapper to allow driver-specific operations like pool resizing.

This PR is a break out from #4202, see comment there for the design input for this PR.

[alexlarsson]

To test this i added:

diff --git a/daemon/graphdriver/devmapper/driver.go b/daemon/graphdriver/d
index e958ef3..430b07f 100644
--- a/daemon/graphdriver/devmapper/driver.go
+++ b/daemon/graphdriver/devmapper/driver.go
@@ -5,6 +5,7 @@ package devmapper
 import (
        "fmt"
        "github.com/dotcloud/docker/daemon/graphdriver"
+       "github.com/dotcloud/docker/engine"
        "github.com/dotcloud/docker/utils"
        "io/ioutil"
        "os"
@@ -140,3 +141,13 @@ func (d *Driver) mount(id, mountPoint string) error {
 func (d *Driver) Exists(id string) bool {
        return d.Devices[id] != nil
 }
+
+func (d *Driver) test(job *engine.Job) engine.Status {
+       fmt.Printf("****** TEST ******** %v\n", job.Env())
+       return engine.StatusOK
+}
+
+func (d *Driver) Install(eng *engine.Engine) error {
+       eng.Register("devmapper:test", d.test)
+       return nil
+}

And running docker devmapper:test -o foo=bar -o "key=value 2" test1 test2
Printed:

[d0d056e0] +job devmapper:test(test1, test2)
****** TEST ******** &[foo=bar key=value 2]
[d0d056e0] -job devmapper:test(test1, test2) = OK (0)

[shykes]

#assignee=shykes

[alexlarsson]

@shykes ping?

[shykes]

@alexlarsson sorry I got overrun. I would like to get this in for 0.11.

@vieux would you mind taking over? (you can steal the PR to confirm).

Thanks

[vieux]

#assignee=vieux

[alexlarsson]

rebased to latest master

[alexlarsson]

Updated to latest master. Any chance someone could take a look now?

[alexlarsson]

I'm closing this in favour of #4202, which contains the same commits. Its just too painful to keep rebasing both, and the dm ops allow you to test the core code.