Add iso_download task

The task accept both file and directory URLs for download an ISO image.
By specifying a directory the task will look for SUM files to fetch the
ISO filename, this is useful to grab the latest ISO image.
Merge pull request #39 from elyezer/iso-download-task

Add iso_download task
Create a task specific for the provisioning job

Rename task reservation_install to provision_install in order to
transform it to be specific to the provisioning job.

Improve the task to be able to clean the VM before each installation.
And add option to install from an ISO image.

Also make iso_install task atomic. Subscribe and install_prerequisites
should be done outside the iso_install task.
Merge pull request #40 from elyezer/provisioning-job-task

Create a task specific for the provisioning job