Cleanup shell's authentication check.

The original logic for whether shell had enough information to
authenticate a user was confusing and will be very difficult to extend
to other forms of authentication.

Change-Id: I88763a651e494e60070a30f0824505acb09310cb
Provide keystone CLI man page.

Resolves: bug 1185142
Implements blueprint: clients-man-pages

Provide basic but hopefully useful man page.

shell.rst was merged into and replaced by the man page in HTML docs.

Change-Id: I0a90e4638cc7e4ed9c02c5abd7d84a4079e5f50d
Merge "Cleanup shell's authentication check."
Merge "Provide keystone CLI man page."