Fixed a bug when the Heat template was not deleted on Env deletion

API now passes an empty 'applications' list as part of environment instead
of a Null-environment

Also, the incomplete 'destroy' section of the Instance class has been removed
This needs to be fixed later, however the current version is fine, as the stack
will be properly deleted by the environment deploy code

Change-Id: I176d5026ade2b0092381b0b2f9bb6273ec21abf7
Closes-bug: #1308910