Fix mounting cacerts.pem
Fix mounting cacerts.pem

Hello!
rancher fails to start when mount path of `cacerts.pem` is `/etc/rancher/ssl`
```log
2019/02/26 14:31:02 [INFO] Listening on /tmp/log.sock
2019/02/26 14:31:02 [INFO] Rancher version v2.2.0-rc1 is starting
2019/02/26 14:31:02 [INFO] Rancher arguments {ACMEDomains:[] AddLocal:auto Embedded:false KubeConfig: HTTPListenPort:80 HTTPSListenPort:443 K8sMode:auto Debug:false NoCACerts:false ListenConfig:<nil> AuditLogPath:/var/log/auditlog/rancher-api-audit.log AuditLogMaxage:10 AuditLogMaxsize:100 AuditLogMaxbackup:10 AuditLevel:0}
I0226 14:31:02.487331       5 http.go:110] HTTP2 has been explicitly disabled
2019/02/26 14:31:02 [FATAL] open /etc/rancher/ssl/cert.pem: not a directory
```
cjellick/rancher,rancher/rancher,rancher/rancher,rancher/rancher,cjellick/rancher,cjellick/rancher,rancher/rancher,rancherio/rancher,rancherio/rancher