User email is optional and can be absent

Change-Id: Idff0f7c33bcaee29c7d1e7be415eb6b5b29b5b1a
Related-Bug: #1260423
Fix parse_isotime filter into horizon

    parse_isotime was imported from oslo to horizon and became a filter,
    but it can raise an exception. If it occurs the template in which
    we use the filter will be broken. A fix has been made that allows the
    filter to fail silently and display a configurable default value.
    Some tests are now performed on this filter. According to Django
    defaultfilters nothing is logged when the evaluation fails.

Fixes bug #1253161

Change-Id: If44cde62c37371c5de7b7f81860f368f61b066bc
Merge "User email is optional and can be absent"