default CFOAuthClient to "cf" if empty
default CFOAuthClient to "cf" if empty

If these fields are not in the config cf auth will fail.

[#132225811]

Signed-off-by: Nick Wei <397a225b6adb51a440bd3821475520403c3e9564@pivotal.io>

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli