fix(settings): allow URI_STUB to be set in the environment to set its value on-the-fly

[macintacos]

This change would allow a user to do something like the following, just before making the API call, allowing them to specify the URI_STUB manually for endpoints that accept something other than v1.5 (for example:
https://www.mongodb.com/docs/atlas/reference/api-resources-spec/#operation/returnOneAdvancedClusterFromOneProject)

You could then do something like this to change the value before making the call:

import os

os.environ["URI_STUB"] = "/api/atlas/v1.5"
atlas.Clusters.get_single_cluster(cluster_name) # would now be using a different URI_STUB behind the scenes

To set if back, the user can just clear the environment variable, or manually revert it to "/api/atlas/v1.0".

[macintacos]

This is very much a bandaid - a proper solution would restrict what values are allowed by a user (I think only v1.0 and v1.5 would be valid in the Atlas API today anyway).

[mgmonteleone]

@macintacos , looking at how to implement this now..

[mgmonteleone]

@macintacos , have updated the PR with a few fixes, but would like to discuss if we should go with this workaround, or just go ahead and do the full fix. Since this workaround will cause the full fix to be breaking, and will require a new major version.