Set cmd_env for trusty and xenial only

[murraybd]

Only set the command environment for the supported releases which require it.

[murraybd]

I was installing the cassandra charm on bionic with the following set in /etc/apt/apt.conf.d/95-juju-proxy-settings:

Acquire::http::Proxy "http://squid.internal:3128";
Acquire::https::Proxy "http://squid.internal:3128";

This was failing with the following Traceback:

2021-02-18 23:27:33 INFO juju-log Failed executing 'add-apt-repository --yes ppa:cassandra-charmers/stable'. Will retry in 10 seconds
2021-02-18 23:27:44 ERROR juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
    hookenv._run_atstart()
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charmhelpers/core/hookenv.py", line 1312, in _run_atstart
    callback(*args, **kwargs)
  File "/var/lib/juju/agents/unit-cassandra-0/charm/reactive/apt.py", line 117, in configure_sources
    keys_var='install_keys')
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charmhelpers/fetch/__init__.py", line 148, in configure_sources
    add_source(source, key)
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charmhelpers/fetch/ubuntu.py", line 600, in add_source
    fn(*m.groups())
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charmhelpers/fetch/ubuntu.py", line 641, in _add_apt_repository
    cmd_env=env_proxy_settings(['https']))
  File "/var/lib/juju/agents/unit-cassandra-0/.venv/lib/python3.6/site-packages/charmhelpers/fetch/ubuntu.py", line 748, in _run_with_retries
    result = subprocess.check_call(cmd, env=env)
  File "/usr/lib/python3.6/subprocess.py", line 311, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['add-apt-repository', '--yes', 'ppa:cassandra-charmers/stable']' returned non-zero exit status 1.

My change resolved the Traceback I encountered and the install was able to complete.

[thedac]

This looks good to me. Given the git history this returns the behavior to pre commit 0f198e4 and still handles < bionic.