v2.3.4

Add 'pre-runner-script' option (#154)

Add a 'pre-runner-script' option to specify arbitrary bash commands to run
before the runner starts up on the instance.  This is useful if you need
to install dependencies or have OS-specific setup steps.  For example:

      - name: Start EC2 runner
        with:
	 mode: start
	 ec2-image-id: ami-07bdb09c49774f92e
	 pre-runner-script: |
	      sudo yum update -y && \
	      sudo yum install docker git libicu -y \
	      sudo systemctl enable docker

Signed-off-by: Tony Hutter <hutter2@llnl.gov>