[IM] Use ifName directly when generting MRTG configurations

The schema original stored ifDescription (as switchport.name) requiring a
per vendor translation function for generating MRTG configurations:

    IXPManager::Utils::switchporttosnmpidentifier($switchport, $vendor);

However, we now also store the ifName directly (which is what MRTG
authors consider "the only sensible way to reference the interfaces of
your switches.

This commit updates the scripts (and sql views) to use ifName directly
including a new utility function to escape characters such as ':',
'@', ' ' and '&' as required by MRTG.
Merge pull request #20 from inex/im-mrtg-polling

[IM] Use ifName directly when generting MRTG configurations