tox: avoid tox testenv subsvars for xenial support

[blackboxsw]

Proposed Commit Message

Commit cd752df added substvars in tip which causes tracesbacks
on Xenial because tox 2.3.1 still suffers from
tox-dev/tox#208.

As a result we cannot use testenv substvars if we want to support
running tox on xenial systems.

Drop the {[testenv:integration-tests]*} substitutions to avoid
tracebacks when running tox -e xenial-dev which has the signature:

"No support for the %s substitution type" % sub_type)
tox.ConfigError: ConfigError: No support for the posargs
substitution type

Additional Context

None

Test Steps

cat > doit.sh <<EOF
git clone https://github.com/canonical/cloud-init
apt-get update
apt-get install tox -y
cd cloud-init
tox -e xenial-dev
EOF
lxc launch ubuntu-dialy:xenial test-x
lxc file push doit.sh test-x/
lxc exec test-x -- bash  /doit.sh


# note exception

Apply this patch.
re-run tox -e xenial-dev

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly N/A
• I have updated or added any documentation accordingly N/A

[OddBloke]

Thanks!