Export certificate 'ca_name' of signed certificates

[KaiHofstetter]

• It exports 'ca_name' instead of the 'ca' value if the certificate is not a ca.
• It exports 'ca' value if the certificate is a ca.

PR https://github.com/cloudfoundry-incubator/credhub-cli/issues/84

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/171596676

The labels on this github issue will be updated when the story is started.

[linux-foundation-easycla[bot]]

The committers are authorized under a signed CLA.

• :white_check_mark: Kai Hofstetter (aba1d672c607cedaf527fc0e3fb43170c3e491a7)