Open Stack

A spec has been raised to add a config option to allow operators to 
choose whether to use the new convergence engine for stack operations. 
For some context you should read the spec first [1]

Rather than doing this, I would like to propose the following:
* Users can (optionally) choose which engine to use by specifying an 
engine parameter on stack-create (choice of classic or convergence)
* Operators can set a config option which determines which engine to use 
if the user makes no explicit choice
* Heat developers will set the default config option from classic to 
convergence when convergence is deemed sufficiently mature

I realize it is not ideal to expose this kind of internal implementation 
detail to the user, but choosing convergence _will_ result in different 
stack behaviour (such as multiple concurrent update operations) so there 
is an argument for giving the user the choice. Given enough supporting 
documentation they can choose whether convergence might be worth trying 
for a given stack (for example, a large stack which receives frequent 
updates)

Operators likely won't feel they have enough knowledge to make the call 
that a heat install should be switched to using all convergence, and 
users will never be able to try it until the operators do (or the 
default switches).

Finally, there are also some benefits to heat developers. Creating a 
whole new gate job to test convergence-enabled heat will consume its 
share of CI resource. I'm hoping to make it possible for some of our 
functional tests to run against a number of scenarios/environments. 
Being able to run tests under classic and convergence scenarios in one 
test run will be a great help (for performance profiling too).

If there is enough agreement then I'm fine with taking over and updating 
the convergence-config-option spec.

[1] 
https://review.openstack.org/#/c/152301/2/specs/kilo/convergence-config-option.rst