lava_results_app: convert Decimal objects to string storing as YAML

Convert Decimal objects to Python strings and then create a regular
YAML ScalarNode to avoid storing Python object types in the results.

Without this patch, YAML text case metadata measurements from the job
results would contain things like this:

    measurement: !!python/object/apply:decimal.Decimal ['15']

which is not standard YAML and can't be imported by non-Python
parsers.  A similar issue can be seen in CSV exports.

With this patch applied, we get this instead:

    measurement: '15.0000000000'

which can be parsed by any standard YAML parser.

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>

lava_results_app/dbutils.py

@@ -42,6 +42,13 @@
 from lava_common.timeout import Timeout
 
 
+def yaml_decimal_str(dumper, value):
+    return yaml.ScalarNode(tag=u'tag:yaml.org,2002:str', value=str(value))
+
+
+yaml.add_representer(decimal.Decimal, yaml_decimal_str)
+
+
 def _check_for_testset(result_dict, suite):
     """
     The presence of the test_set key indicates the start and usage of a TestSet.