Fix InternalException raised on stack-show

Getting attributes of a OS::Nova::Server raises a 500 error if the server
doesn't exist. So stack show doesn't work. We should catch the exception
and return the default values of the output keys of the server while
getting attributes to make sure the api works.

Change-Id: If569b4d73abbce93d3c56c45a1eb284720c6e416
Closes-Bug: #1279085
Default properties to the empty dict in ResourceGroup

Sets the properties of the grouped resource to {} when it's None to make
sure the resource is built properly.

Change-Id: I2938c741063159ef81a7996d203adaf466cf19c4
Closes-Bug: #1291421
Merge "Fix InternalException raised on stack-show"
Merge "Default properties to the empty dict in ResourceGroup"