Add a try to install_prerequisites ping checks

Sometimes the ping -c1 `hostname -s` fails, but when ssh to the machine
and running the same command it works. Add another try after 5 seconds
if the ping check fails.
Merge pull request #32 from elyezer/prerequisites-ping

Add a try to install_prerequisites ping checks
Improve README

Add examples on how to use tasks to manage virtual machines, install
satellite and nightly builds and how to specify a ssh key when running
fabric.
Merge pull request #34 from elyezer/improve-readme

Improve README
Add vm_list_base task

The vm_list_base task will list all available base images. Update README
to show how to use the vm_list_base task.

Also improve the vm_list to only show running virtual machines by
default.
Merge pull request #35 from elyezer/vm-list

Add vm_list_base task
Fix repository headline in README

Related to #37
Clean up README.md

* Fix a broken link to robottelo.
* Add instructions for installing prerequisites.
* Wrap all text at 80 characters.
Merge pull request #36 from Ichimonji10/readme

Clean up README.md
Enable Python 3 compatibility

Make the StringIO import conditional. Add parentheses to print statements.
Refactor function "subscribe"

Try to make the function easy to read, and do make the function
pylint-compliant.
Merge pull request #38 from Ichimonji10/refactor-and-compat

Enable Py 3 compat, refactor function
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