[#584] load settings from environment without a config file present

[d-w-moore]

See issue description. Basically this change means that you won't have to declare a config file path in the environment (even if that value is null length, denoting the default path of ~/.python_irodsclient) in order to get the environmental overrides to work.

[alanking]

Seems fine. Does it make sense to include a test for this?

[d-w-moore]

Seems fine. Does it make sense to include a test for this?

It does indeed.

[alanking]

Test looks good to me. Had a couple of fortification suggestions and then I think this should be good to go.

[alanking]

Just one last thing about that skip and I'm happy with it

[d-w-moore]

Responding to #585 (comment) @alanking 's comment. above (Sorry I cannot respond int eh conversation above, but I guess GitHub works in strange ways.)

The os.path.absdir may not be needed, as :

1. it would only be needed for Python2, for which we soon plan to drop support
2. the only eventuality that would cause the join's result to be inaccurate in finding the target script is an invocation of os.chdir by another test, and this will not happen, since setupssl.py is the only thing calling chdir, and its only caller (login_auth_test.py) has now been banished from the test suite.

[alanking]

Alright, let's get this squashed, @d-w-moore. Omit #s for now.

[d-w-moore]

Squashed!

[alanking]

# it

[d-w-moore]

Done.