Make CLI index init idempotent

[fenekku]

This issue stems from https://github.com/inveniosoftware/invenio-cli potentially re-running invenio index init multiple times under the covers and not wanting to throw an exception when the index is already created.

Like Ansible, invenio-cli's goal (and by extension this cli commands' goal) is to get to a state rather than perform a certain action.