Propagate environment variables to workers

It is common place to use environment variables to configure security credentials such as AWS keys. When spawning a remote cluster, for example using `SSHCluster` or other remote `SpecCluster` derivatives it could be useful to propagate some or all environment variables from the parent Python process.

This could be implemented as a flag and would build on the work in #2508.

It could be a boolean

```python
with SSHCluster(..., propagate_env=True) as cluster:
...
```

or a list of envs to propagate

```python
with SSHCluster(..., propagate_env=['AWS_ACCESS_KEY_ID', 'AWS_SECRET_ACCESS_KEY', 'AWS_REGION']) as cluster:
...
```

or something else...

Thoughts?