provider/manual: default enable-os-refresh-update to true

The manual provider should be doing "apt-get update" and
installing packages by default.

Fixes https://bugs.launchpad.net/juju-core/+bug/1363130
Merge pull request #643 from axw/lp1363130-manual-default-os-update

provider/manual: default enable-os-refresh-update to true

The manual provider should be doing "apt-get update" and
installing packages by default.

Fixes https://bugs.launchpad.net/juju-core/+bug/1363130