dev: don't require root to build a test environment (part 1)

Building a test environment shouldn't require root, as already
discussed: https://github.com/crowbar/crowbar/pull/1651

Anyway, setting group ownership is unnecessary and also beyond the
scope of ./dev.
dev: don't require root to build a test environment (part 2)

Building a test environment shouldn't require root, as already
discussed: https://github.com/crowbar/crowbar/pull/1651

Also, installation of package dependencies is currently beyond the
scope of ./dev (that's why we have test_prereqs_installed() rather
than something called install_prereqs()).  Therefore it doesn't make
sense to call linux-install.sh which is currently an Ubuntu-specific
script which only installs erlang dependencies and nothing else.

Dependencies are correctly documented in doc/devguide/, and if we want
to automate their installation, it should be handled properly for all
barclamps via a separate script which consumes metadata from (say)
each barclamp's crowbar.yml.  Then that script can *optionally* be
called from ./dev when required.
build_lib.sh: refactor output functions and add warn()
doc/devguide: flesh out and align lists of dev dependencies
dev: make test_prereqs_installed() more helpful

When one or more dependencies are missing, list them all,
and redirect to a URL linking to instructions on how to
install them.