The `get_sshca_server_url` function encapsulates the logic of
identifying the URL of the SSHCA server. For now, it only considers the
`SSHCA_SERVER` environment variable, but eventually, it will also
support other configuration methods like a configuration file. Moving
this to a separate function will allow other areas of the code to share
the same logic.
Dustin
c26d67a25b