Split `module/vault' from `module/util'

The `vault' module is now the dedicated secret management module.

This new module has also been enabled in `libsite.sh'.
Updated `unit/static-gpg.sh': fix, scaffolding

Renamed `PROFILE' to `SITE_PROFILE' which was overlooked in the last
overhaul commit of `libsite.sh'.

Added setUp/tearDown scaffolding functions for the gpg module.
Updated `module/unit': per-module setUp/tearDown
Unit tests for `module/util': nima/site#4
Updated `.travis.yml'

Install from `share/unit/apt.conf' to prevent user-interactivity.

Install from `share/unit/rng-tools' to speed up RNG; this doesn't
currently work in the Travis CI environment for some reason.
Updated `README.md': module devel status table