bump minimal ansible version to 2.8.0

[LuckySB]

/kind bug

What this PR does / why we need it:

ansible 2.7.x fail with error

 {"msg": "The conditional check '{{ 'bootstrap-os' not in ansible_play
_role_names }}' failed. The error was: error while evaluating conditional ({{ 'bootstrap-os' not in ansible_play_role_names }}): Unable to look up a name or a
ccess an attribute in template string ({{ 'bootstrap-os' not in ansible_play_role_names }}).\nMake sure your variable name does not contain invalid characters
 like '-': argument of type 'StrictUndefined' is not iterable\n\nThe error appears to have been in '/srv/kubespray/roles/kubespray-defaults/tasks/main.yaml':
line 9, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n# do not run gather facts w
hen bootstrap-os in roles\n- name: set fallback_ips\n  ^ here\n"}

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[Miouge1]

Since requirements.txt is Ansible 2.9 we might want to use that for minimum version?

Might be nice to have a variable similar to keep the minimum version only in 1 place?

[LuckySB]

Since requirements.txt is Ansible 2.9 we might want to use that for minimum version?

Might be nice to have a variable similar to keep the minimum version only in 1 place?

I think that it is necessary to diff between the minimum and recommended version.
I upgraded the version of ansible to 2.8, because 2.7 fail with an error.
Many people install ansible not through pip

[Miouge1]

Thank you @LuckySB

/lgtm