default ha_proxy.ssl_pem "" is invalid

In haproxy job, there is an `ha_proxy.ssl_pem` property with a default value of "". Unforuntately, this is an invalid property value and the haproxy job will no start.

```
==> /var/vcap/sys/log/haproxy/startup_stderr.log <==
[ALERT] 333/173110 (1764) : parsing [/var/vcap/jobs/haproxy/config/haproxy.config:26] : 'bind :443' : unable to load SSL private key from PEM file '/var/vcap/jobs/haproxy/config/cert.pem'.
[ALERT] 333/173110 (1764) : parsing [/var/vcap/jobs/haproxy/config/haproxy.config:34] : 'bind :4443' : unable to load SSL private key from PEM file '/var/vcap/jobs/haproxy/config/cert.pem'.
[ALERT] 333/173110 (1764) : Error(s) found in configuration file : /var/vcap/jobs/haproxy/config/haproxy.config
[ALERT] 333/173110 (1764) : Proxy 'https-in': no SSL certificate specified for bind ':443' at [/var/vcap/jobs/haproxy/config/haproxy.config:26] (use 'crt').
[ALERT] 333/173110 (1764) : Proxy 'ssl-in': no SSL certificate specified for bind ':4443' at [/var/vcap/jobs/haproxy/config/haproxy.config:34] (use 'crt').
[ALERT] 333/173110 (1764) : Fatal errors found in configuration.
Sat Nov 30 17:31:10 UTC 2013: Errored starting HAProxy

==> /var/vcap/sys/log/haproxy/startup_stdout.log <==
Sat Nov 30 17:31:10 UTC 2013: Starting HAProxy
Sat Nov 30 17:31:10 UTC 2013: Errored starting HAProxy
```