helm-hosted: the order of "version" in index.yaml should be most recent to oldest

ISSUE:

The default version used by helm clients seems to pick the first version in the index.yaml (eg. Rancher, Lens).
Also, by looking at some index.yaml used in the remote site (eg. Bitnami), it's also sorted by the "version" as semver.
But the index.yaml generated by Nexus looks like alphabetical order (old one first), so that users need to be careful to install package from Nexus helm hosted repository.

REPRODUCE STEPS:

1. Create a helm-hosted repo
2. Download test packages:

   curl -O https://charts.bitnami.com/bitnami/node-11.4.13.tgz
   curl -O https://charts.bitnami.com/bitnami/node-11.4.15.tgz

3. Upload above two package (order doesn't matter)

   curl -v -u admin:admin123 "http://localhost:8081/repository/helm-hosted/" -T node-11.4.13.tgz
   curl -v -u admin:admin123 "http://localhost:8081/repository/helm-hosted/" -T node-11.4.15.tgz

4. Check the http://localhost:8081/repository/helm-hosted/index.yaml

EXPECTED BEHAVIOUR:

Newer version (11.4.15) should be the top of index.yaml.

ACTUAL BEHAVIOUR

Older version (11.4.13) shows in the top, so that when user try to install this "node", the default version is set to 11.4.13:

POTENTIAL CAUSE:

Currently, it seems the order is decided by the index set on the "asset" class in OrientDB, as Nexus does not sort in CreateIndexServiceImpl#buildIndexYaml, and just use the result of "select from asset where (component IS NOT NULL AND attributes.helm.asset_kind = 'HELM_PACKAGE') and (bucket=xxxx)" (no order specified)