[release-2.27] Add `manual` option to the `external_cloud_provider` variable (#11884)

* Add `manual` option in the `external_cloud_provider` value

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

* Update external cloud provider description in roles & sample inventory

Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>

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Signed-off-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>
Co-authored-by: ChengHao Yang <17496418+tico88612@users.noreply.github.com>