Added filter operators

In this changeset, I refactored the DB filter system to support
comparison operators using a django-like syntax.
A filter can take 2 forms:

- "<FIELDNAME>" which is a syntactic sugar for "<FIELDNAME>__eq"
- "<FIELDNAME>__<OPERATOR>" where <OPERATOR> is the comparison operator
  to be used.

Here is the list of the supported operators:

- 'eq' (==)
- 'neq' (!=)
- 'gt' (>)
- 'gte' (>=)
- 'lt' (<)
- 'lte' (<=)
- 'in' (in)
- 'notin' (not in)

Change-Id: I53a61d50a3253342a40f0ff87cb5612ed57a3bd1

watcher/common/exception.py

@@ -150,6 +150,11 @@ class InvalidIdentity(Invalid):
     msg_fmt = _("Expected a uuid or int but received %(identity)s")
 
 
+class InvalidOperator(Invalid):
+    msg_fmt = _("Filter operator is not valid: %(operator)s not "
+                "in %(valid_operators)s")
+
+
 class InvalidGoal(Invalid):
     msg_fmt = _("Goal %(goal)s is invalid")