dev: ensure gems are available for data bags validation

Certain gems are required for data bag validation, so we need to
ensure they are installed before performing validation.  We want
to avoid hardcoding those dependencies in ./dev, since that violates
DRY (they're already in Gemfile) and they may later change.  So
we just move validation after the existing bundle install.  It's
also necessary to run via bundle exec.
doc/devguide: manual gem install should no longer be necessary
doc/devguide/Ubuntu-dev-env.md: ensure erlang is installed
dev: add check for erlang being installed
dev: avoid creating a backup file called 'Gemfilee'

sed's -i option takes an optional suffix for creating backup files,
so you can't bundle the -i and -e options together.
update-git.sh: fix rsync to match new structure under /tmp/crowbar-dev-test
add support for Gemfile.d and routes.d plugin mechanisms
dev: don't relocate Gemfile

61a43b1c copied the Gemfile to the root of the test installation, and
1c60c571 changed the copy to a move.  However, this relocation breaks
Rails conventions, doesn't match what happens outside the test
environment, and creates extra complications e.g. with the rsync in
travis-ci/update-git.sh, and introduces uncertainty over whether
Gemfile.lock and Gemfile.d should also be relocated or not.

It's perfectly easy to ensure that you're in the correct directory
before running bundle exec, or simply passing --gemfile=... to bundle,
so let's take this conventional approach instead, and keep the Gemfile
within the Rails app to which it's coupled.