[release-1.9] Add root configuration API.

[istio-testing]

Manual cherrypick required.

#1848 failed to apply on top of branch "release-1.9":

Applying: Add root configuration API.
Using index info to reconstruct a base tree...
M	mesh/v1alpha1/config.pb.go
M	mesh/v1alpha1/config.proto
M	mesh/v1alpha1/istio.mesh.v1alpha1.gen.json
M	mesh/v1alpha1/istio.mesh.v1alpha1.pb.html
Falling back to patching base and 3-way merge...
Auto-merging mesh/v1alpha1/istio.mesh.v1alpha1.pb.html
CONFLICT (content): Merge conflict in mesh/v1alpha1/istio.mesh.v1alpha1.pb.html
Auto-merging mesh/v1alpha1/istio.mesh.v1alpha1.gen.json
Auto-merging mesh/v1alpha1/config.proto
Auto-merging mesh/v1alpha1/config.pb.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Add root configuration API.
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".