Add RHEL support

[NVitanovic]

This pull request resolves issue #16 and allows the Vulmap Linux script to work on RHEL based systems that utilize rpm package manager.

The following changes have been done:

• Increase the timeout to 240 seconds since Vulnmon API is responding slower.
• Added a function for extracting base distro type.
• Added rpm command in the same format as dpkg in the getProductList().

The remaining flow of the script has not been modified.
Other package managers can be added in the same way based on a different distro if they follow /etc/os-release convention.

[ozelfatih]

Hi @NVitanovic ,

Thanks for your contribution. I tested your script but it didn't give a satisfying result because Vulmon API doesn't have a separate DB for Redhat.

Before merging the code, we must do some development on Vulmon API. For that reason, I am not merging it for now. We have plans to add Redhat support to API in the following months btw.