Allow seaf-cli to start if $HOME isn't defined

Currently, seaf-cli will refuse to start if the HOME environment variable isn't defined, because `DEFAULT_CONF_DIR` is set using `os.environ['HOME']`. This makes it so seaf-cli can't be started (by default) from non-login shells, even if `--confdir` is passed.

This pull request makes --confdir a required argument if `$HOME` is undefined, allowing `seaf-cli` to be started from non-login shells.