add description tag from ifAlias

[pdion891]

Added description tag from ifAlias for network interfaces.

This might be an ugly piece of code to implement the tag. If you have a cleaner code, cancel this PR and enable the tag if possible.

tested with OpenTSDB 2.2

Regards,

[coveralls]

Coverage increased (+0.2%) to 77.82% when pulling 7405d7c on pdion891:multiprocess into 98ad149 on frogmaster:multiprocess.

[frogmaster]

I think it would be better to use separate resolver class for this. For example subclass IfName class IfAlias(IfName) and add it to setup.py "ifname_ifalias=opentsdb.snmp.resolvers.ifname:IfAlias", or something along those lines...