helm: explicitly configure helm chart base url

Currently helm index configures URLs to charts with relative path
(see https://ceph.github.io/csi-charts/index.yaml).

Artifactory contains support for virtual repositories. Virtual
repositories are able to cache remote repositories and provide
offline access to those repositories.

In order for this to work correctly, charts must specify absolute base URL to
artifacts.

This commit adds this base URL using argument for helm index command.

URL with previous approach:
```yaml
    urls:
    - cephfs/ceph-csi-cephfs-3.13.0.tgz
```

URL with current approach (my personal "lirt" repo was used to test this, the
MR specifies correct "ceph" path)
```yaml
    urls:
    - https://lirt.github.io/csi-charts/cephfs/ceph-csi-cephfs-3.13.0.tgz
```

Signed-off-by: Ondrej Vasko <ondrej.vaskoo@gmail.com>

deploy.sh

@@ -84,7 +84,7 @@ push_helm_charts() {
 	popd >/dev/null
 
 	pushd "${CHARTDIR}/csi-charts/docs" >/dev/null
-	helm repo index .
+	helm repo index . --url "https://ceph.github.io/csi-charts/"
 	git config user.name "${GITHUB_USER}"
 	git config user.email "${GITHUB_EMAIL}"
 	git add --all :/ && git commit -m "Update for helm charts ${PACKAGE}-${VERSION}"