fixing update related bug and adding support for clean_install

According to the ossec unattended installation docs, setting USER_UPDATE or USER_UPDATE_RULES to any value will be interpreted as true.  So the preloaded-vars.conf.erb template has been modified to omit these instead of setting them to "n".

Because of this these attributes cannot explicitly be set to false.  So, there is also a USER_CLEANINSTALL option which support has been added for in the preloaded-vars.conf.erb template.

From http://ossec-docs.readthedocs.org/en/latest/manual/installation/install-source-unattended.html :

    # If USER_UPDATE is set to anything, the update
    # installation will be done.
    #USER_UPDATE="y"

    # If USER_UPDATE_RULES is set to anything, the
    # rules will also be updated.
    #USER_UPDATE_RULES="y"