Swift backend: add application credential support

[kayrus]

This PR adds an application credential auth support to the swift backend. Application credentials auth is already supported by an openstack provider. See https://github.com/terraform-providers/terraform-provider-openstack/pull/642

/cc @jtopjian , @jbardin

[kayrus]

cc @radeksimko. I hope you'll find time to review this PR

[jtopjian]

This LGTM. The functionality is equivalent to the OpenStack provider which has been confirmed to work.

[pselle]

@kayrus Thanks so much for this contribution -- it generally looks good, but at present Terraform is feature-frozen pending the 0.12 release. Again, thank you, and appreciate the contribution, and looking forward to getting to it after 0.12.

[kayrus]

@pselle I'm afraid that users will be confused using application credentials auth along with the swift backend. They will observe that Openstack provider works fine, but not the backend.

[kayrus]

rebased on top of #18671
@apparentlymart, please review

[ghost]

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