support python3, smaller fixes, work on Fedora for casl

[ericzolf]

What does this PR do?

basically small changes I needed to get casl running on my laptop:

• Select correct selinux python library depending on python version (hence the name of the branch)
• Add check_mode false to force listing VMs even in check mode (check mode still doesn't go through but at least further)
• Add a note that defaults variables must be overwritten in the host's inventory (and not on the VM's one)
• Make the ISO unmount handler work by replacing with_items with loops (might also be a Python 3 vs. 2 thingy, but the solution should work also for Python 2)
• Make virt-install command generation more readable by breaking it across multiple lines.

How should this be tested?

Just check that there is no regression on RHEL 7 / Python 2 (no new feature)

Is there a relevant Issue open for this?

n/a see GChat discussion on #casl

Other Relevant info, PRs, etc.

n/a

People to notify

cc: @redhat-cop/infra-ansible

[ericzolf]

I've added two smaller fixes:

1. avoid mounting the ISO at boot time, as it might break the boot process if the ISO or the directory has disappeared (happened to me),
2. there is no reason to disable the Apache process if it was enabled before, hence removed (and the description of the task was anyway wrong).

[oybed]

Overall all good changes - thank you. A few nitpick/questions inline.

[oybed]

LGTM