introduce app diego flag to app model

this is set iff the CF_DIEGO_RUN_BETA environment variable is set, and is not
exposed through the API. this allows us to perform more efficient queries in
the new bulk apps endpoint (where we only want to include diego apps).

Signed-off-by: Simon Leung <leungs@us.ibm.com>
don't blow up if the diego column goes away when saving the app

...as will happen some time in the future.

Signed-off-by: Simon Leung <leungs@us.ibm.com>
only include diego apps in the new bulk endpoint

Signed-off-by: Simon Leung <leungs@us.ibm.com>