ntp unit tests broken if no package program available in test environment

[Scott Moser]

apply this diff to show the error, just make 'ntp_installable' not
find a package installer. At minimum, we just need to mock that out.

$ git diff
diff --git a/cloudinit/config/cc_ntp.py b/cloudinit/config/cc_ntp.py
index 15ae1ecd..5ebdd461 100644
--- a/cloudinit/config/cc_ntp.py
+++ b/cloudinit/config/cc_ntp.py
@@ -147,6 +147,7 @@ def ntp_installable():
     if util.system_is_snappy():
         return False

+ return False
     if any(map(util.which, ['apt-get', 'dnf', 'yum', 'zypper'])):
         return True

$ tox-venv py3 python3 -m nose tests/unittests/test_handler/test_handler_ntp.py:TestNtp
...EEEEEE............
======================================================================
ERROR: Test ntp handler renders the shipped distro ntp.conf templates.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 272, in test_ntp_handler_real_distro_templates
    cc_ntp.handle('notimportant', cfg, mycloud, None, None)
  File "/home/smoser-public/src/cloud-init/cloud-init/cloudinit/config/cc_ntp.py", line 128, in handle
    write_ntp_config_template(ntp_cfg, cloud, confpath, template=template_name)
  File "/home/smoser-public/src/cloud-init/cloud-init/cloudinit/config/cc_ntp.py", line 204, in write_ntp_config_template
    "not rendering %s"), path)
RuntimeError: ('No template found, not rendering %s', '/etc/systemd/timesyncd.conf.d/cloud-init.conf')

======================================================================
ERROR: Ntp schema validation allows for an empty ntp: configuration.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 305, in test_ntp_handler_schema_validation_allows_empty_ntp_config
    with open(ntp_conf) as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/ci-TestNtp.nmgu_ja4/ntp.conf'

======================================================================
ERROR: Ntp schema validation warns of invalid keys present in ntp config.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 374, in test_ntp_handler_schema_validation_warns_invalid_key_present
    with open(ntp_conf) as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/ci-TestNtp.zj7sj7n8/ntp.conf'

======================================================================
ERROR: Ntp schema validation warns of non-strings in pools or servers.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 331, in test_ntp_handler_schema_validation_warns_non_string_item_type
    with open(ntp_conf) as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/ci-TestNtp.agut528j/ntp.conf'

======================================================================
ERROR: Ntp schema validation warns of duplicates in servers or pools.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 400, in test_ntp_handler_schema_validation_warns_of_duplicates
    with open(ntp_conf) as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/ci-TestNtp.ykjfhbx7/ntp.conf'

======================================================================
ERROR: Ntp schema validation warns of non-array pools or servers types.
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/smoser-public/src/cloud-init/cloud-init/tests/unittests/test_handler/test_handler_ntp.py", line 352, in test_ntp_handler_schema_validation_warns_of_non_array_type
    with open(ntp_conf) as stream:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/ci-TestNtp.4f3ycmp1/ntp.conf'

----------------------------------------------------------------------
Ran 21 tests in 0.085s

FAILED (errors=6)

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3021