Support snmp check on windows

There is two things in here, which I probably should have split up but I did at the same time:

- Fixing the dependencies requirements to put them at the same level than for the self contained agent.
- Addition of pysnmp to the windows agent dependencies to support the SNMP check on windows. We also bundle the standard mibs in the python format to avoid it being a real pain to setup. The import of pysnmp_mibs before the packaging in setup.py is due to the way the mibs are moduled. If it's not done, these mibs are not going to be included so it's necessary.