Switching local updates to default of true.

We don't want local installs of Juju to become outdated, so unless the user has specified otherwise, we will default this value to true.
Merge pull request #941 from katco-/lxc-apt-update-to-true

Switching local updates to default of true.

We don't want local installs of Juju to become outdated, so unless the user has specified otherwise, we will default this value to true.