allow ssh authentication with username and password

[leej35]

Add --username and --password arguments for explicit username and password-based ssh authentication. (yet does not support host-specific authentication but same username/password for across all hosts)

[wookayin]

Thanks for your contribution. But why not SSH? Password is not good for security.

[leej35]

For some machines (like I use), public key-based authentication is not possible due to some unknown issues. Especially when I don't have a sudo account, it's very hard to debug/fix the problem -- so I made a slight modification on gpustat-web to allow explicit user id / password based authentication.