roachtest: add overrides to perturbation/* tests

Allow the user to override perturbations by specifying the environment
variable `PERTURBATION_OVERRIDE`. This applies to all perturbation
tests, but is useful for reproducing specific metamorphic failures where
it is unclear which parameter change caused the difference.

An example of using an override to modify the test block size:

```
PERTURBATION_OVERRIDE=blockSize=1024 roachtest run
perturbation/full/addNode
```

Epic: none

Release note: None

pkg/cmd/roachtest/tests/perturbation/framework.go

@@ -13,8 +13,10 @@ import (
 	"io"
 	"math"
 	"math/rand"
+	"os"
 	"reflect"
 	"sort"
+	"strconv"
 	"strings"
 	"time"
 
@@ -278,6 +280,16 @@ func (v variations) perturbationName() string {
 
 // finishSetup completes initialization of the variations.
 func (v variations) finishSetup() variations {
+	// Apply any environment variable overrides first.
+	if overrides, found := os.LookupEnv("PERTURBATION_OVERRIDE"); found {
+		for _, override := range strings.Split(overrides, ",") {
+			parts := strings.Split(override, "=")
+			if err := v.applyEnvOverride(parts[0], parts[1]); err != nil {
+				panic(fmt.Sprintf("can't apply override: %s: %v", override, err))
+			}
+		}
+	}
+
 	for k, val := range v.acMode.getSettings() {
 		v.clusterSettings[k] = val
 	}
@@ -286,6 +298,64 @@ func (v variations) finishSetup() variations {
 	return v
 }
 
+// applyEnvOverride applies a single override to the test.
+func (v *variations) applyEnvOverride(key string, val string) (err error) {
+	switch key {
+	case "fillDuration":
+		v.fillDuration, err = time.ParseDuration(val)
+	case "blockSize":
+		v.maxBlockBytes, err = strconv.Atoi(val)
+	case "perturbationDuration":
+		v.perturbationDuration, err = time.ParseDuration(val)
+	case "validationDuration":
+		v.validationDuration, err = time.ParseDuration(val)
+	case "ratioOfMax":
+		v.ratioOfMax, err = strconv.ParseFloat(val, 64)
+	case "splits":
+		v.splits, err = strconv.Atoi(val)
+	case "numNodes":
+		v.numNodes, err = strconv.Atoi(val)
+	case "numWorkloadNodes":
+		v.numWorkloadNodes, err = strconv.Atoi(val)
+	case "vcpu":
+		v.vcpu, err = strconv.Atoi(val)
+	case "disks":
+		v.disks, err = strconv.Atoi(val)
+	case "mem":
+		v.mem = spec.ParseMemCPU(val)
+		if v.mem == -1 {
+			err = errors.Errorf("unknown memory setting: %s", val)
+		}
+	case "leaseType":
+		for _, l := range leases {
+			if l.String() == val {
+				v.leaseType = l
+				return nil
+			}
+		}
+		return errors.Errorf("unknown lease type: %s", val)
+	case "cloud":
+		for _, c := range cloudSets {
+			if c.String() == val {
+				v.cloud = c
+				return nil
+			}
+		}
+		return errors.Errorf("unknown cloud: %s", val)
+	case "acMode":
+		for _, a := range admissionControlOptions {
+			if a.String() == val {
+				v.acMode = a
+				return nil
+			}
+		}
+		return errors.Errorf("unknown admission control mode: %s", val)
+	default:
+		return errors.Errorf("unknown key: %s", key)
+	}
+	return err
+}
+
 func addMetamorphic(r registry.Registry, p perturbation) {
 	rng, seed := randutil.NewPseudoRand()
 	v := p.setupMetamorphic(rng)