This corrects an issue where if a symbolic link was given to the
`certs:add` command, an error would be thrown even though the file was
valid.
Issue: #4251
The 'server.key' and 'server.crt' can printed to stdin.
The two options are:
`dokku certs:show <app> key > server.key`
`dokku certs:show <app> crt > server.crt`
This will create two files, `server.key` and `server.crt`.
Those files contain the private key and the certificate.
Includes tests.
