The bundle path was previously stored on the `config.Root` type under the
assumption that the first configuration file being loaded would set it. This is
slightly counterintuitive and we know what the path is upon construction of the
bundle. The new location for this property reflects this.

PR #604 added functionality to load a bundle without a `databricks.yml` if both
the `DATABRICKS_BUNDLE_ROOT` and `DATABRICKS_BUNDLE_INCLUDES` environment
variables were set. We never ended up using this in downstream tools so this
can be removed.

This is incompatible with the function signature of Cobra's `PreRunE`
functions, so this change updates any use of PreRunE to be inlined in the main
run function.