stream-ssl: Read existing CA certificate more eagerly during bootstrap.

stream-ssl: Read existing CA certificate more eagerly during bootstrap.

When do_ca_cert_bootstrap() attempts to bootstrap a CA certificate from a
remote host, it gives up if the CA certificate file already exists.  It
knows that this file did not exist some time earlier (because it checked),
so it logged a warning and just returns.  The next time that
stream_ssl_set_ca_cert_file() gets called, it will read the new CA
certificate file and all will be well.

That works OK in ovsdb-server, which calls stream_ssl_set_ca_cert_file()
every time through its main loop.  It does not work well for ovs-vswitchd,
which only calls that function when it needs to reconfigure.  But it
should work fine to call it directly from do_ca_cert_bootstrap(), so this
commit changes it to do that.

Bug #2635.