Set default -Authentication to Negotiate

[jborean93]

I know the contributor section says PRs aren't accepted and to open an issue for an RFC but this was quite a simple fix so I decided to open the PR instead.

On Windows the absence of -Authentication for the PSSession WSMan parameters means to use Negotiate auth. On Linux if you omit the the -Authentication parameter then it will fail with the error

Enter-PSSession: MI_RESULT_ACCESS_DENIED

This PR just sets the default value for -Authentication to be the same as -Authentication Negotiate to match the same behaviour on Windows.