Disable tracer option

Disables the function tracer on the command line
I have a pet peeve about asking for not the thing you don't want...
```diff
diff --git a/cfme/fixtures/tracer.py b/cfme/fixtures/tracer.py
index 24f94f6..7e1d3ac 100644
--- a/cfme/fixtures/tracer.py
+++ b/cfme/fixtures/tracer.py
@@ -129,13 +129,14 @@ depths = default_depths
 
 
 def pytest_addoption(parser):
-    parser.addoption("--no-tracer", action="store_true", default=False,
+    # stored as config.options.tracer, and True (default) if tracer is enabled
+    parser.addoption("--no-tracer", dest='tracer', action="store_false", default=True,
                      help="Disable the function tracer")
 
 
 def pytest_runtest_call(__multicall__, item):
     """hook to run each test with traced function calls"""
-    if not item.config.getvalue('no_tracer'):
+    if item.config.getoption('tracer'):
         with function_trace.trace_on(to_trace,
                                      depths=depths,
                                      tracer=function_trace.PerThreadFileTracer(
```