Fix private key authentication with dbshell

[timgraham]

Fixes #67

(Pending the release of snowflake-connector-python 3.6.0 which supports the new options.)

[timgraham]

@sfc-gh-hachouraria snowflake-connector-python 3.6.0 was just released. Could you test this?

[sfc-gh-hachouraria]

@sfc-gh-hachouraria snowflake-connector-python 3.6.0 was just released. Could you test this?

Thank you for making these changes @timgraham,

I just tried this branch (dbshell-private-key) and I can confirm that the private_key_file and private_key_file_pwd options work in all cases (usual database interaction from the app and commands such as python3 ./manage.py migrate, as well as the SnowSQL shell launch from python3 ./manage.py dbshell).

Django's general config validation appears to require that I still specify the PASSWORD option so I placed a bogus value in it during these tests.

[timgraham]

Thanks for raising the issue about an inappropriately required PASSWORD. I added a commit to address it.

[sfc-gh-hachouraria]

Thanks for raising the issue about an inappropriately required PASSWORD. I added a commit to address it.

Thank you! I had no idea internal configs already existed for skipping that requirement — Django is brilliant.

Confirming I can completely skip PASSWORD in the config now with this change.