Improve password storage

[geoffklee]

Currently, the default behaviour is to store the JSS credentials in a plain text file (the python-jss module's preferences file). It would be much better to integrate with keychain, or at least allow an option to prompt for a password every time.

[geoffklee]

Looks like the keyring module will handle this just fine. Running it in a virtualenv, I get the same error as here:

jaraco/keyring#219

but the suggested fix of re-signing the virtualenv's python binary seems to solve the problem.

[geoffklee]

I've had a bash at implementing this in a branch: https://github.com/gkluoe/git2jss/compare/keychain

It tries to do The Right Thing and defaults to using the keychain.

If it finds a password in the plist file (left by an older version), it will offer to move it into the keychain and if the user refuses, will only operate if the '--no-keychain' option is specified.

[geoffklee]

I decided it's probably better to use our own plist file for git2jss, rather than using the default. This keeps things neater.

Due to what I think is a bug in python-jss, I've ended up copying almost the entire JSSPrefs class into the KJSSPrefs subclass, which doesn't seem ideal, but it works.

[geoffklee]

Fixed in https://github.com/gkluoe/git2jss/releases/v0.1.0