Add swh.objstorage.objstorage_azure.AzureCloudObjStorage

Removed swh.objstorage.objstorage_cloud.AzureCloudObjStorage
implementation which does not work for now.
(Related issue LIBCLOUD-851 opened issue on apache-libcloud's bug
report)

Prerequisite:

• pip3 install azure-storage (no debian package for it yet)

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