Bump ansible requirement to 2.8.0

[tangrufus]

#1141 bases on ansible/ansible-lint#457 (comment)
which depends on ansible/ansible#51030 which merged into anisble v2.8.0.

See: https://discourse.roots.io/t/provisioning-server-not-working-because-ansible-ssh-extra-args-is-undefined/17484

[swalkinshaw]

I think we have some 2.7 compat code which would be nice to get rid of too. It's probably under lib but I don't remember exactly.

[tangrufus]

Never worked with python. No idea where to start.
Can you help? @swalkinshaw

[swalkinshaw]

Main thing I remember is this:

trellis/lib/trellis/plugins/callback/vars.py

Lines 27 to 32 in bf0cf3b

	# handle Ansible 2.7 and 2.8 cases by normalizing each into a dict
	try:
	from ansible import context
	self._options = context.CLIARGS
	except ImportError:
	self._options = vars(cli.options) if cli else {}

I believe with 2.8 we can just rely on context existing. That might be the only compat code actually

[swalkinshaw]

Updated in #1166