Correct the order of assembly deletion

If a heat stack has not been created (the case in some functional tests)
deployer.delete_heat_stack() is fast and ends up deleting the assembly
before the assembly handler has a chance to set the state to DELETING.

This patch just sets the state to DELETING before calling delete_heat_stack()
to prevent this.

Change-Id: I2ba64c02f1d797327582b93229eb980617c3a504
Closes-bug: #1342399
Merge "Correct the order of assembly deletion"