Support explicitly specified artifacts e.g. fat JARs

[enolan]

What did you implement:

Fixes #89.

How did you implement it:

We check whether the serverless.variables.service.package.artifact variable exists. If it does, it gets attached to every function object. When a function is invoked, if the artifact variable is set, the artifact is unpacked with the unzip shell command to a temporary directory .sls-simulate-unzipped. That directory is mounted into the docker container at /var/task. Otherwise the project root is mounted there.

How can we verify it:

You can pick any JVM Serverless project, whichever is easiest for you. Here's instructions for Scala/SBT

• sls create -t aws-scala-sbt -p foo
• Replace serverless.yml with this:

service: scala-template
provider:
  name: aws
  runtime: java8
package:
  artifact: target/scala-2.12/hello.jar
functions:
  hello:
    handler: hello.ApiGatewayHandler
    events:
      - http:
          path: hello
          method: get
plugins:
  - serverless-plugin-simulate

• sbt assembly - You'll need to have SBT installed
• sls simulate apigateway
• in another terminal curl http://localhost:3000/hello

You should see this:

Serverless: Creating event
Serverless: Invoking hello
Serverless: Invoking function hello.ApiGatewayHandler
WARNING: Your kernel does not support swap limit capabilities or the cgroup is not mounted. Memory limited without swap.
START RequestId: 347d188f-702f-4838-82c5-620ad9367f17 Version: $LATEST
END RequestId: 347d188f-702f-4838-82c5-620ad9367f17
REPORT RequestId: 347d188f-702f-4838-82c5-620ad9367f17	Duration: 230.17 ms	Billed Duration: 300 ms	Memory Size: 1024 MB	Max Memory Used: 24 MB	

{"statusCode":200,"body":"Go Serverless v1.0! Your function executed successfully!","headers":{"x-custom-response-header":"my custom response header value"},"base64Encoded":true}
Serverless: Mapping response
GET /hello 200 1125.335 ms - 56

Without this PR you'll get the error from #89.

Todos:

• [x] Write tests
• [x] Write documentation - bugfixes don't need documentation
• [x] Fix linting errors
• [x] Provide verification config/commands/resources
• [x] Change ready for review message below

Is this ready for review?: YES