ENH: separate auth data loading into separate class.

To simplify the authentication logic in enstaller, we don't try to ask for
user input when getting authentication information, and we don't write
back authentication information outside the 'enpkg --userpass' mode.

Strictly speaking, get_auth is thus not backward-compatible, in the case
where authentication has not been set up, or changed between the time
the config file has been loaded and get_auth gets called.
FEAT: add an is_auth_configured function.
ENH: authenticate early in enpkg.main, and assume authentication from there.
BUG: fix 2.6.
STY: update following review comment.
REF: remove obsolete/unused function argument.
Merge pull request #155 from enthought/consolidate_auth

Consolidate auth