parse_isotime filter to parse a date string.

This then allows the resulting date value to be piped
to one of the standard django date/time format filters

This is identical to oslo timeutils parse_isotime, but annotated with
@register.filter

Required for blueprint: heat-ui

Change-Id: Ibecca77d50bb5c0df59e07e9572069215229f60a
Merge "parse_isotime filter to parse a date string."