fix: Always use single datasource if specified (#5098)

This change may require a user to add `None` to the `datasource_list`
defined in `/etc/cloud/cloud.cfg[.d]` if they have a customized
datasource_list and want the DataSourceNone fallback behavior.

ds-identify would automatically append "None" to the datasource_list
if a single entry was provided in /etc/cloud/cloud.cfg[.d].
This wasn't a problem in the past as the python code would detect
a single datasource along with None as an indication to automatically
use that datasource. Since the python code no longer does that,
we should ensure that one specified datasource results in one specified
datasource after ds-identify has run.

Fixes GH-5091

tests/unittests/test_ds_identify.py

@@ -622,7 +622,7 @@ def test_single_entry_defines_datasource(self):
  mydata = copy.deepcopy(VALID_CFG["Ec2-hvm"])
  cfgpath = "etc/cloud/cloud.cfg.d/myds.cfg"
  mydata["files"][cfgpath] = 'datasource_list: ["NoCloud"]\n'
- self._check_via_dict(mydata, rc=RC_FOUND, dslist=["NoCloud", DS_NONE])
+ self._check_via_dict(mydata, rc=RC_FOUND, dslist=["NoCloud"])
 
  def test_configured_list_with_none(self):
  """When datasource_list already contains None, None is not added.

tools/ds-identify

@@ -1999,7 +1999,11 @@ _main() {
  # if there is only a single entry in $DI_DSLIST
  if [ $# -eq 1 ] || [ $# -eq 2 -a "$2" = "None" ] ; then
  debug 1 "single entry in datasource_list ($DI_DSLIST) use that."
- found "$@"
+ if [ $# -eq 1 ]; then
+ write_result "datasource_list: [ $1 ]"
+ else
+ found "$@"
+ fi
  return
  fi