modify dict implementation, fix typo

[dallen5]

• closes Setting environment variables on remote cluster does not work #64
• tests added / passed
• docs reflect changes
• passes flake8 rhg_compute_tools tests docs
• entry in HISTORY.rst

This updates the handling of the env_items parameter in get_cluster(), changing the dictionary unpacking to for k, v in env_items.items() instead of for k, v in env_items.values().

Also, this fixes a typo in the deprecated list-of-dicts implementation (from isintance to isinstance).

Finally, the docstring is updated to match the dict implementation rather than the deprecated list-of-dicts implementation.

[delgadom]

Jeez. How did this ever work? Looks great - thanks so much!

[delgadom]

The CI failures look like they're likely an issue with a new version of pytest. I've triggered a build on master to see if we get the same issue.

[delgadom]

Have a fix in #67. We can merge this in once that is merged into master

[delgadom]

Note this PR is waiting on dallen5#2

[dallen5]

@delgadom I merged in those changes, looks like this is passing tests (which seem to have sped up 5-10x??)

[delgadom]

sweet thank you!