netsnmptestenv: Explicitly remember used TCP ports in instance variables

This way test_netsnmptestenv.py can inspect in a future-safe manner whether
the net-snmp daemons are still running and occupy the TCP ports or have
exited properly. Currently the ports are still hard-coded but might become
dynamic in a future commit.
netsnmptestenv: Always strip() SNMP command's output immediately
netsnmptestenv: Rename __del__ to shutdown

As done for netsnmpagent in e7d9144c and for the same reasons.
netsnmptestenv: Move shutdown()'s process killing into own function

Sooner or later our net-snmp test environment will consist of more than
snmpd (eg. snmptrapd), so it makes sense to generalize the process killing.
netsnmptestenv: Have snmpcmd() raise exceptions in timeout/"no such OID" cases

The SNMP commands executed might hit a timeout (no running snmpd, wrong
community string etc.) or a "no such OID" condition (no subagent running).
These cases must be detectable by tests without having to check the output.
Use nose as testing framework

The advantages over unittest are quite well-known (eg. not having to create
test classes, test discovery etc.), so I won't elaborate in much detail here.
Add docstrings to test cases in test_netsnmptestenv.py
Have test_netsnmptestenv.py also test shutdown(), not just call it
Use @timed decorator to test the time test_netsnmptestenv functions use
Make use of nicer/more effective nose tools in test_netsnmptestenv.py
Test that snmpget really fails again after netsnmptestenv shutdown
Add more tests for complete and clean shutdown() in test_netsnmptestenv.py

Tests that
- check that the snmpd has really exited by killing it's remembered PID
- check that the tmpdir used by the testenv has been removed
Use slightly more intelligent process killing in netsnmptestenv module

Instead of hammering the process to be killed with signals continously,
sleep small amounts of time and check procfs. This is obviously
Linux-specific, but so far I have not heard of anyone trying to use
python-netsnmpagent with net-snmp on different platforms.