If enabled, allow users and orgs to set their time machine expiration

Fixes https://www.pivotaltracker.com/story/show/142881203

util/config/validators/validate_timemachine.py

@@ -0,0 +1,25 @@
+import logging
+
+from util.config.validators import BaseValidator, ConfigValidationException
+from util.timedeltastring import convert_to_timedelta
+
+logger = logging.getLogger(__name__)
+
+class TimeMachineValidator(BaseValidator):
+  name = "time-machine"
+
+  @classmethod
+  def validate(cls, config, user, user_password):
+    try:
+      convert_to_timedelta(config['DEFAULT_TAG_EXPIRATION']).total_seconds()
+    except ValueError as ve:
+      raise ConfigValidationException('Invalid default expiration: %s' % ve.message)
+
+    if not config['DEFAULT_TAG_EXPIRATION'] in config['TAG_EXPIRATION_OPTIONS']:
+      raise ConfigValidationException('Default expiration must be in expiration options set')
+
+    for ts in config['TAG_EXPIRATION_OPTIONS']:
+      try:
+        convert_to_timedelta(ts)
+      except ValueError as ve:
+        raise ConfigValidationException('Invalid tag expiration option: %s' % ts)