Add support for specifying applications to install on cluster

[jbeemster]

Are these applications that are usually installed using Jobflow Steps?

[BenFradet]

FYI, we're using Elasticity's applications in EmrEtlRunner:

• here
• and here

which seem to be added to the payload here:

https://github.com/rslifka/elasticity/blob/master/lib/elasticity/job_flow.rb#L221

[alexanderdean]

Assigning to Ben 😄