[wsrp] [CR306] - Add default cacheControl

[Rich Thompson <richt2@us.ibm.com>]

Document: Spec
Section: 5.1.11
Requested by: Subbu Allamaraju

Old Text:
New Text:   [O] CacheControl    cacheControl

Reasoning: We currently allow Producers to return cacheControl along with
markup so that the Consumer can try to cache the markup. However, the Consumer
will not know a priori whether a portlet's markup can be cached or not.
Some portlet programming models such as JSR168 allow portlet developers
to specify a default cache interval with the portlet's metadata. In the
current protocol, there is no means for exposing this metadata to Consumers
at discovery time in an interoperable manner.

By adding this field to the PortletDescription we can align WSRP with JSR168
more tightly.

Errata? no