Add option to force the vulnerability detection in unsupported OS

[crolopez]

This feature allows to force the detection of agents with unsupported operating systems by checking their packages with the supported OVALs.

To do this we have to add the "allow" tag in the feed you want to make compatible with more operating systems. As content we will add the operating systems separated by commas as in the following example:

<feed name="REDHAT-7">
   <disabled>yes</disabled>
   <allow>linux mint-18, ubuntu-16</allow>
   <update_interval>1h</update_interval>
</feed>

[vikman90]

This PR depends on PR #470