heat engine : map DBInstance DBSecurityGroups parameter correctly

heat engine : map DBInstance DBSecurityGroups parameter correctly

The DBInstance nested stack template defines a DBSecurityGroups list parameter,
which should be "CommaDelimitedList" type, curently it's a "List" type which
breaks when the template is parsed.  To make this work we also have to mangle
the property (passed in the top level template) format to match the parameter
format, because in AWS Lists are not represented in the same way for Properties and
Parameters (sigh..)

fixes bug 1096099

Change-Id: Ie078d61847d89ea9dcd55f798b808e595c2d2e12
Signed-off-by: Steven Hardy <shardy@redhat.com>