so-iso-build: remove /var/ossec/etc/sslmanager*

[dougburks]

sslmanager.cert and sslmanager.key need to be removed before the ISO is built. They will be regenerated when the user installs the ISO and boots for the first time.

[dougburks]

commit:
Security-Onion-Solutions/securityonion-iso@7ed4d7a

[dougburks]

submitted for testing:
https://groups.google.com/d/topic/security-onion-testing/y3dmOD3G9-g/discussion

[dougburks]

Published:
https://blog.securityonion.net/2018/10/securityonion-iso-20151016.html