Refactor kube exec to support io streams

[tdmanv]

Change Overview

Breaks up kube.ExecWithOptions into sync and async parts.

Pull request type

Please check the type of change your PR introduces:

• Work in Progress
• Refactoring (no functional changes, no api changes)
• Trival/Minor
• Bugfix
• Feature
• Documentation

Issues

• Continuous logs from spawned commands #231

Test Plan

/go/src/github.com/kanisterio/kanister # go test -v ./pkg/function -check.v -check.f Exec
=== RUN   Test
time="2019-09-04T03:41:04Z" level=info msg="Pod: test-deployment-f4f76-6f6955dfd7-t4wd6 Container: test-container Out: hello world echo hello world"
PASS: kube_exec_all_test.go:106: KubeExecAllTest.TestKubeExecAllDeployment      1.907s
time="2019-09-04T03:41:05Z" level=info msg="Pod: test-statefulset-twkbh-0 Container: test-container Out: hello world echo hello world"
PASS: kube_exec_all_test.go:140: KubeExecAllTest.TestKubeExecAllStatefulSet     1.733s
time="2019-09-04T03:41:10Z" level=info msg="Pod: kubeexectesttestkubeexec-0 Container: test-container Out: hello world echo hello world"
PASS: kube_exec_test.go:133: KubeExecTest.TestKubeExec  3.373s
PASS: kube_exec_test.go:169: KubeExecTest.TestParseLogAndCreateOutput   0.001s
OK: 4 passed
--- PASS: Test (10.35s)
PASS
ok      github.com/kanisterio/kanister/pkg/function     10.400s

• Manual
• Unit test
• E2E