Remove partial clean-up in Instance.destroy

Instance class implementation is based on OS::Nova::Server
resource in Heat and related networking resources. When deploy
called on Instance number of resources are allocated in Heat.

Some classes (e.g. descendants of Instance) can allocate other
resources that depends on presence of resources allocated by Instance,
for example HeatSWConfigInstance allocate OS::Heat::SoftwareDeployment
that depends on underlying OS::Nova::Server.

When Instance is destroyed and clean-up stage is finished underlying
resources are removed from stack. But since some resources still depend
on removed ones whole stack becomes invalid and deployment fails.

This change implements quick workaround by removing resource clean-up
in Instance class. Warning, this change introduce resource leak, cause
resoures are allocated during Instance deployment now will be deleted
only when whole Environment (and therefor stack) will be deleted.

Change-Id: I772f0b35918cd554fcbe1e830766ec23d5641176
Closes-Bug: #1359998
Merge "Remove partial clean-up in Instance.destroy"