[FEATURE] Add config option for fallocate_reserve

[Diko Parvanov]

In order to prevent disks from filling up, add the option to configure fallocate_reserve parameter in account-server.conf, container-server.conf, and object-server.conf, as described in https://docs.openstack.org/swift/latest/admin_guide.html#preventing-disk-full-scenarios.

[Andrea Ieri (aieri)]

Fix proposed in https://review.opendev.org/#/c/726944/

[Andrea Ieri (aieri)]

Subscribing field-medium since we need this urgently for a customer.
We have a merge proposal ready, we only need a review and a merge.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/726944
Committed: https://git.openstack.org/cgit/openstack/charm-swift-storage/commit/?id=d31095d482b83a48f3f1724a48856b09cc2d5903
Submitter: Zuul
Branch: master

commit d31095d482b83a48f3f1724a48856b09cc2d5903
Author: Drew Freiberger <email address hidden>
Date: Mon May 11 15:18:32 2020 -0500

    Add config option for fallocate_reserve

    In order to prevent disks from filling up, add the option to configure
    fallocate_reserve parameter in account-server.conf,
    container-server.conf, and object-server.conf, as described in
    https://docs.openstack.org/swift/latest/admin_guide.html#preventing-disk-full-scenarios

    This change adds the config option file-allocation-reserve with a
    default of 0.

    Change-Id: Ib396a151250bb8d1733f9b5b9cab7eb506c9f6c6
    Closes-Bug: 1872069