remove overwrite from initProvider-forProvider merge

[lsviben]

Description of your changes

This resolves a TODO left from #237 where during the spec.initProvider to spec.forProvider merge the fields in spec.initProvider would overwrite the spec.forProvider ones, instead
of skipping those fields.

Although we would expect and suggest not to set the same fields for both, it still good to fix this behaviour.

To merge the 2, we used mergo, with the option SliceDeepCopy to merge
the slices from initProvider to forProvider.

err = mergo.Merge(&params, initParams, mergo.WithSliceDeepCopy)

Looking deeper, we can see the the mergo.WithSliceDeepCopy also sets the Overwrite to true

So simply setting the overwrite to false does the trick.

err = mergo.Merge(&params, initParams, mergo.WithSliceDeepCopy, func(c *mergo.Config) {
	c.Overwrite = false
})

Fixes #240

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable to ensure this PR is ready for review.
• [] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Extended the unit test to test for overwrites.

[ulucinar]

Thanks @lsviben, lgtm.
Looks like we need to fix the reporting of some tests in pkg/terraform/files_test.go but this is not a blocker for this PR. E.g.,

diff --git a/pkg/terraform/files_test.go b/pkg/terraform/files_test.go
index ec38af8..353fd31 100644
--- a/pkg/terraform/files_test.go
+++ b/pkg/terraform/files_test.go
@@ -488,8 +488,8 @@ func TestWriteMainTF(t *testing.T) {
                        if err = json.JSParser.Unmarshal([]byte(tc.want.maintf), &wantJson); err != nil {
                                t.Errorf("cannot unmarshal want main.tf.json: %v", err)
                        }
-                       if diff := cmp.Diff(res, wantJson, test.EquateConditions()); diff != "" {
-                               t.Errorf("\n%s\nWriteMainTF(...): -want error, +got error:\n%s", tc.reason, diff)
+                       if diff := cmp.Diff(wantJson, res); diff != "" {
+                               t.Errorf("\n%s\nWriteMainTF(...): -want main.tf.json, +got main.tf.json:\n%s", tc.reason, diff)
                        }
                })
        }