Fix set flag for write-values

[vixus0]

The --set flag for write-values is currently a no-op. I'm guessing this is because we can't simply pass the flag through to helm like we do for the other commands.

I've worked around this by using Helm's own strvals package to parse the values provided by --set and then merging the resulting YAML with the generated values.

My attempt at a test is failing due to a nil pointer dereference. I'm not very good at debugging Go yet, so any help would be appreciated!

[mumoshu]

@vixus0 Thanks for the PR! I'm eager to merge this once the following error gets resolved:

--- FAIL: TestWriteValues_WithSetFlag (0.01s)
panic: unexpected error: exec: "helm": executable file not found in $PATH [recovered]
	panic: unexpected error: exec: "helm": executable file not found in $PATH

goroutine 3140 [running]:
testing.tRunner.func1.1(0x1c54b00, 0xc00077cba0)
	/usr/local/go/src/testing/testing.go:940 +0x421
testing.tRunner.func1(0xc0008c1200)
	/usr/local/go/src/testing/testing.go:943 +0x600
panic(0x1c54b00, 0xc00077cba0)
	/usr/local/go/src/runtime/panic.go:975 +0x3e3
github.com/roboll/helmfile/pkg/helmexec.Output(0xc00023b1e0, 0xc000adac60, 0x1, 0x1, 0xc00041a800, 0x32, 0x40, 0x203000, 0x0)
	/home/circleci/workspace/helmfile/pkg/helmexec/runner.go:83 +0x1007
github.com/roboll/helmfile/pkg/helmexec.ShellRunner.Execute(0x0, 0x0, 0xc0009980f0, 0x1f02643, 0x4, 0xc00089b020, 0x3, 0x3, 0x0, 0xc00089b020, ...)
	/home/circleci/workspace/helmfile/pkg/helmexec/runner.go:39 +0x1af
github.com/roboll/helmfile/pkg/helmexec.getHelmVersion(0x1f02643, 0x4, 0x2c2ed00, 0xc000b80120, 0xc000648e00, 0xc000adaf98, 0x7f91d3a1dc28, 0x0, 0xc000b80120)
	/home/circleci/workspace/helmfile/pkg/helmexec/exec.go:93 +0x15b
github.com/roboll/helmfile/pkg/helmexec.New(0x1f02643, 0x4, 0xc0009980f0, 0x1f06212, 0x7, 0x2c2ed00, 0xc000b80120, 0x0)
	/home/circleci/workspace/helmfile/pkg/helmexec/exec.go:104 +0x78
github.com/roboll/helmfile/pkg/app.(*App).getHelm(0xc0008c1680, 0xc0007edb80, 0x0, 0x0)
	/home/circleci/workspace/helmfile/pkg/app/app.go:637 +0x47b
github.com/roboll/helmfile/pkg/app.(*App).visitStatesWithSelectorsAndRemoteSupport.func1(0xc0007edb80, 0xc000000001, 0xa3, 0x0, 0xc0007ed900)
	/home/circleci/workspace/helmfile/pkg/app/app.go:896 +0x8f
github.com/roboll/helmfile/pkg/app.(*App).visitStates.func1(0x1f0d7e9, 0xd, 0xc0005829d0, 0x8, 0x0, 0x0)
	/home/circleci/workspace/helmfile/pkg/app/app.go:736 +0xb6c
github.com/roboll/helmfile/pkg/app.(*App).visitStateFiles.func1(0x37ddfa8, 0xc000000001)
	/home/circleci/workspace/helmfile/pkg/app/app.go:570 +0x19f
github.com/roboll/helmfile/pkg/app.(*App).within(0xc0008c1680, 0x1f01415, 0x1, 0xc0005b17e0, 0xc0005b17a0, 0x2)
	/home/circleci/workspace/helmfile/pkg/app/app.go:512 +0x94c
github.com/roboll/helmfile/pkg/app.(*App).visitStateFiles(0xc0008c1680, 0x1f0d7e9, 0xd, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/circleci/workspace/helmfile/pkg/app/app.go:564 +0x604
github.com/roboll/helmfile/pkg/app.(*App).visitStates(0xc0008c1680, 0x1f0d7e9, 0xd, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/circleci/workspace/helmfile/pkg/app/app.go:648 +0x15a
github.com/roboll/helmfile/pkg/app.(*App).visitStatesWithSelectorsAndRemoteSupport(0xc0008c1680, 0x1f0d7e9, 0xd, 0xc00035acb0, 0xc00035ad20, 0x1, 0x1, 0x1, 0xc0005829c0)
	/home/circleci/workspace/helmfile/pkg/app/app.go:903 +0x8e9
github.com/roboll/helmfile/pkg/app.(*App).ForEachState(0xc0008c1680, 0xc00035ad30, 0xc00035ad20, 0x1, 0x1, 0x438cc0, 0xc00035ad50)
	/home/circleci/workspace/helmfile/pkg/app/app.go:784 +0x110
github.com/roboll/helmfile/pkg/app.(*App).WriteValues(0xc0008c1680, 0x2c551a0, 0xc00089bb60, 0xc00089bb60, 0xc0009980f0)
	/home/circleci/workspace/helmfile/pkg/app/app.go:240 +0xe9
github.com/roboll/helmfile/pkg/app.TestWriteValues_WithSetFlag(0xc0008c1200)
	/home/circleci/workspace/helmfile/pkg/app/app_test.go:4265 +0x536
testing.tRunner(0xc0008c1200, 0x1f6a7c0)
	/usr/local/go/src/testing/testing.go:991 +0x1ec
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1042 +0x661
FAIL	github.com/roboll/helmfile/pkg/app	367.655s

[vixus0]

Ok, after taking a deeper look at this today, I think it's not possible to test this using the TestFs like the other tests because of the use of temporary files. For the time being I've switched things over to using bytes.Buffer instead of temporary files. Hopefully it hasn't broken things anywhere else 🙈

[vixus0]

I've reverted the buffer change and added an integration test instead - there are too many moving parts for this to be tested with a unit test, I think.