Properly parse Gravity 5.0.36 status

[wadells]

Description

This PR fixes a Gravity 5.0.x compatiblity regression introduced by #233. Specifically, when trying to parse the output of gravity status --output=json on 5.0.36 robotest would fail with the following error: json: cannot unmarshal string into Go struct field.

Risk Profile

• Bug fix (patch release)

Related Issues

Fixes #247.

Testing Done

See the unit tests added. Here is what it looks like to run them:

go test ./infra/gravity before the second commit

walt@work:~/git/robotest$ go test ./infra/gravity/
--- FAIL: TestGravity5036ActiveStatusValidation (0.00s)
    gravity_test.go:127: 
                Error Trace:    gravity_test.go:127
                Error:          Received unexpected error:
                                json: cannot unmarshal string into Go struct field ClusterStatus.cluster.system_status of type int
                Test:           TestGravity5036ActiveStatusValidation
    gravity_test.go:130: 
                Error Trace:    gravity_test.go:130
                Error:          Received unexpected error:
                                expected system_status 1, found 0
                Test:           TestGravity5036ActiveStatusValidation
--- FAIL: TestGravity5036DegradedStatusValidation (0.00s)
    gravity_test.go:147: 
                Error Trace:    gravity_test.go:147
                Error:          Received unexpected error:
                                json: cannot unmarshal string into Go struct field ClusterStatus.cluster.system_status of type int
                Test:           TestGravity5036DegradedStatusValidation
FAIL
FAIL    github.com/gravitational/robotest/infra/gravity 0.013s
FAIL

go test ./infra/gravity after

walt@work:~/git/robotest$ go test ./infra/gravity/
ok      github.com/gravitational/robotest/infra/gravity (cached)

Here is a complete 5.0.36 install: logs

Additional Information

I considered splitting the datamodel, such that GravityStatus became an interface fufilled by two different implementations: Gravity50ClusterStatus and Gravity52ClusterStatus (or the like). I chose not to go with this approach because:

• It is more invasive. It required rewriting every signature and piece of code that consumes GravityStatus or any of its member fields.
• The life expectancy of this code is not that long. 5.0.x has been out of support for over a year (EoS April 13, 2019), and at some point we'll stop testing --upgrade-via tests from this release whereupon all this code can be removed wholesale.