[RFE] Update OpenStack Go lib (`gophercloud`) to its version 2 in Mantle

[tormath1]

Current situation

Mantle uses gophercloud and its "old" version, let's switch to version 2 (v2)

Impact

• Outdated and unmaintained lib
• Miss reading OpenStack credentials from now well known cloud.yaml

Ideal future situation

Mantle test suite and friends use github.com/gophercloud/gophercloud/v2

Implementation options

• Update the lib in Mantle
• While there is compilation issue update codebase
• Run a full CI against an OpenStack provider (devstack or Brightbox at this time)

Additional information

Ideally we deprecate the homemade configuration file in favor of clouds.yaml.