auto-detect chroot

[James Cassell]

For ansible 2.5 and later, we can use the ansible_is_chroot fact to pre-populate rhel7stig_system_is_chroot. We should set defaults:

rhel7stig_system_is_chroot: "{{ ansible_is_chroot | default(False) }}"

Then gather_facts: subset="!all,!min,chroot" when ansible >= 2.5 and ansible_is_chroot not defined.