ssl_ca is not necessary when ssl_cert is signed by
a trusted CA, such as GeoTrust, because a trusted
cert chain is in the system already. Users can just
provide ssl_cert and ssl_key to enable SSL endpoint
in that case.

Closes-Bug: #1711354
Change-Id: I4a34df1a2c2bf5705e02b713d968a22f4bbf57cf