[Reviewer: Matt]  Create SNMP live tests, and add infrastructure for 'non-call' tests

Matt,

This pull request adds SNMP tests to the live test suite, and therefore adds a concept of tests which don't involve calls.

The output is something like:

$ rake test[rkd.cw-ngv.com] TESTS="*SNMP*" SNMP_AGENTS="bono-1.rkd.cw-ngv.com,sprout-1.rkd.cw-ngv.com"
Clearwater SNMP - bono-1.rkd.cw-ngv.com - Passed
CPU/Mem SNMP - bono-1.rkd.cw-ngv.com - Passed
Nonexistent SNMP - bono-1.rkd.cw-ngv.com - Passed
SNMP with wrong community - bono-1.rkd.cw-ngv.com - Passed
Clearwater SNMP - sprout-1.rkd.cw-ngv.com - Failed
  RuntimeError thrown:
   - No results from SNMP for host sprout-1.rkd.cw-ngv.com
     - /home/rkd/cw-src/clearwater-live-test/lib/tests/snmp.rb:27:in `block (2 levels) in <top (required)>'
...