Section class already provides password() method that does the right
thing (store the value in secure.config) and suppresses the input from
the terminal. However this method expects user/password pair to exist.
With OAuth Github authentication provider we need to provide password
for GitHub Client Secret. Add another convenience method for Section to
operate on single field with description.
Change-Id: I9593cd5b35ae263003c1d1c9d771638a780ef96a
9e58478b66