Introduces a new API to allow OpenStack administrators to start
the share manager's ensure shares operation.
Also, introduced a new configuration option named
`update_shares_status_on_ensure`, so administrators can define
whether the shares status should be updated during the ensure
shares operation or not.
A new column was added to the services table, named `ensuring`.
Through this field, we can identify whether there is an undergoing
ensure shares operation or not.
Closes-Bug: #1996793
Partially-Implements: bp ensure-shares-api
Change-Id: If7bf059eb8581f20a3ceb7c1af93558774f4ef5e
101becf9b4