Implement warning-level validation test failures

Sometimes we don't want to fail a battery of validation tests just
because a device implementation misses a small detail (example: cost
of OSPF default route). You can use 'level: warning' on such tests,
indicating that if the test fails it's not catastrophic, but neither
is it good.

docs/netlab/validate.md

@@ -25,6 +25,15 @@ options:
   -e, --error-only      Display only validation errors (on stderr)
 ```
 
+The **netlab validate** command returns the overall test results in its exit code:
+
+| Exit code | Meaning |
+|----------:|---------|
+| 0 | All tests passed |
+| 1 | At least one test failed |
+| 2 | `netlab validate` did not find a single usable test to execute |
+| 3 | Some of the tests generated warnings |
+
 ## Example
 
 **netlab validate** command was executed on the [Advertise IPv4 Prefixes to BGP Neighbors](https://bgplabs.net/basic/3-originate/) lab before the user configured BGP prefix origination:

docs/topology/validate.md

@@ -28,6 +28,7 @@ Each test has a name (dictionary key) and description (dictionary value) -- anot
 * **wait** (integer, optional) -- Time to wait (when specified as the only action in the test) or retry (when used together with other actions). The first wait/retry timeout is measured from when the lab was started; subsequent times are measured from the previous test containing the **wait** parameter.
 * **wait_str** (string, optional) -- Message to print before starting the wait.
 * **stop_on_error** (bool, optional) -- When set to `True`, the validation tests stop if the current test fails on at least one of the devices.
+* **level** (string, optional) -- When set to `warning,` the test failure does not indicate that the whole testing sequence has failed but generates a warning message. 
 
 [^TEX]: Objects, non-empty strings, lists, or dictionaries, integers not equal to zero or `True`.