Initial implementation of the block storage manager presumed that Amazon
provider would only have a single block storage manager and consequently
used the generic class name `Amazon::BlockStorageManager`. @rolivery
suggested to change the name to indicate the actual type of the manager,
which in this case is EBS.

This patch thus replaces all occurrences where `BlockStorageManager` was
used before with `StorageManager::Ebs`.

Signed-off-by: Gregor Berginc <gregor.berginc@xlab.si>

Since we are now explicitly adding EBS provider, this patch changes the
name of the corresponding dependent block storage manager.

Signed-off-by: Gregor Berginc <gregor.berginc@xlab.si>

Following the renaming of the EBS storage manager, this patch also
changes the EBS storage key in the `config/settings.yml` file.

Signed-off-by: Gregor Berginc <gregor.berginc@xlab.si>