Add gathercmd test step

[mimir-d]

• this is a gather type plugin, holds onto all targets and runs the
  given command once; returns a framework error if the command fails
• add to contest generator and server

testing:

• server with resume

> go run . --adminServerAddr "" --resumeJobs --pauseTimeout 10s
[...]
[2022-07-28T22:15:10.605+01:00 D localsetup.go:176] Running command: /usr/bin/sleep 60m attempt=%!s(uint32=0)   job_id=11       run_id=1        step_index=0  step_label=setup

• contestcli

> go run . start -w < ./descriptors/start-literal.json

with

"Steps": [
    {
        "name": "localsetup",
        "label": "setup",
        "parameters": {
            "binary": [
                "sleep"
            ],
            "args": [
                "60m"
            ]
        }
    }
]

Also tested successful setup, failing setup, canceled while running command.

[mimir-d]

last push changes the event model; instead of emitting the same event on all input targets, just use the first one. This makes it more clear that the setup only ran once, instead of per target.

[mimir-d]

todo: add teardown equivalent after #83 is merged. When that happens, anything created by the setup will be uniquely identifier thru output vars and the teardown can act on those artifacts exactly.

[tfg13]

I would name this something like GatherCmd to make it a bit more clear what it does.

LGTM, the only thing is the obvious RCE, which we already discussed and is the same as with the cmd plugin.

[tfg13]

Suggested change

	// New initializes and returns a new Cmd test step.
	// New initializes and returns a new localsetup test step.

[mimir-d]

would name this something like GatherCmd

you know.. that's a good point. I was thinking of adding another local_teardown plugin, but it's not different than this one. I should rename to GatherCmd

[mimir-d]

renamed to GatherCmd

test with false

• out status

{
 "ServerID": "fedora-pc1c0jta",
 "Data": {
  "Status": {
   "Name": "test job",
   "State": "JobStateFailed",
   "StateErrMsg": "Job 18 failed after 32.86962ms: exit status 1",
   "StartTime": "2022-07-29T12:39:30Z",
   "EndTime": "2022-07-29T12:39:30Z",
   "RunStatus": {
    "JobID": 18,
    "RunID": 1,
    "StartTime": "0001-01-01T00:00:00Z",
[...]
	"TargetStatuses": [
       {
        "JobID": 18,
        "RunID": 1,
        "TestName": "Literal test",
        "TestStepName": "GatherCmd",
        "TestStepLabel": "setup",
        "Target": {
         "ID": "1234"
        },
        "InTime": "2022-07-29T12:39:30Z",
        "OutTime": "2022-07-29T12:39:30Z",
        "Error": "",
        "Events": [
         {
          "SequenceID": 137,
          "EmitTime": "2022-07-29T12:39:30Z",
          "Header": {
           "JobID": 18,
           "RunID": 1,
           "TestName": "Literal test",
           "TestAttempt": 0,
           "TestStepLabel": "setup"
          },
          "Data": {
           "Target": {
            "ID": "1234"
           },
           "EventName": "GatherCmdStart",
           "Payload": {
            "Path": "/usr/bin/false",
            "Args": [
             "/usr/bin/false"
            ]
           }
          }
         },
         {
          "SequenceID": 138,
          "EmitTime": "2022-07-29T12:39:30Z",
          "Header": {
           "JobID": 18,
           "RunID": 1,
           "TestName": "Literal test",
           "TestAttempt": 0,
           "TestStepLabel": "setup"
          },
          "Data": {
           "Target": {
            "ID": "1234"
           },
           "EventName": "GatherCmdEnd",
           "Payload": {
            "ExitCode": 1,
            "Err": "stdout: \nstderr: "
           }
          }
         }
        ]
       },