Only update apt cache if required

[colin-nolan]

Updating the apt cache as the first install step is not a great idea: https://github.com/savagegus/ansible-consul/blob/b655bf22a711e865ef52609f7cfc1d97db42cbb2/tasks/install.yml#L2-L6

This is beacuse:

• We might not have to apt install anything yet have to wait for the cache to update/pay for the bandwidth.
• If the cache update, it will count as a change, despite the fact that nothing of interest to this role has been changed.