test: Remove global-setup

The uniqueness constraints in loopback-workspace have been fixed.
it is no longer needed to reset the Memory connector between tests,
Extract loadModels() from acl/index to lib/actions
Extract actions.saveProject, simplify err handling

Extract actions.saveProject as a complement to actions.loadProject.

Modify helpers.reportValidationError() to support arbitrary `err`
argument, including undefined/null. This eliminates duplication of
err value & type checks in callbacks.
property: implement model-property generator

Implement generator for model properties.

Modify model generator to repeatedly ask for model properties and
add them by calling property generator.

Modify actions.loadProject & actions.saveProject to allow generators
to pass an already loaded project when invoking a nested generator.
This allows the "model" generator to defer any file changes until
all properties are defined. When the user interrupts the model generator
prompts with ^C, no files are modified.
datasource: list available connectors via Project

Move the hard-coded list of available connector to loopback-workspace,
use `Project.listAvailableConnectors` to obtain the list.
acl: use AclDefinition and its value providers