Allow deploys with existing role

[wvidana]

This code allows deploying Lambda using an existing role (instead of creating a new one).

• No policy needed
• Use role name

Note: tried to update new test for this functionality, but got some problems (too deep recursion) on placebo package. Anyways, editing the foo/kappa.yml to use a role instead of a policy worked (tests passed).

A kappa.yml example would be:

---
name: kappa-simple
environments:
  dev:
    profile: foobar
    region: us-west-2
    role:
      name: foo-the-role
lambda:    
  description: Foo the Bar
  handler: simple.handler
  runtime: python2.7
  memory_size: 256
  timeout: 3

[wvidana]

Solves #83

[zzsnzmn]

Anything else needed to get this merged? Would be happy to assist.

[josegonzalez]

Needs docs.

[wvidana]

Just added that example on the PR description the the sample code folder