Support the "stateless" parameter for Actors

[mwvaughn]

With the release of the autoscaler in Abaco 1.2.0, it is critical to support the actor stateless parameter. Actors registered as being stateless will automatically be scaled via the auto-scaler, so it is important that this become the default.

This should be implemented as follows:

• In reactor.rc the variable REACTOR_STATEFUL sets project-level behavior
  • Default is empty (REACTOR_STATEFUL=)
• In abaco init a new line for REACTOR_STATEFUL needs to be added to the templated config file
• In abaco create support the -s|-S options to create a stateless or stateful actor
• In abaco deploy support -s|-S > REACTOR_STATEFUL to manage actor statelessness

[mwvaughn]

Implemented in:

• fa4439b
• 950a5d2
• 60b94b1

[mwvaughn]

Reopening to test