Improve DynamicFabric test coverage

- for when fabric has existing children
DynamicFabric: add existing children as members
Adds EntitySpec.groups and EntitySpec.members
Reworks BrooklynRestApiLauncher

Makes it much simpler to launch servers in any context, with whatever
security provider you want, custom filters and with or without the jsgui.
Alters logging in BrooklynPropertiesSecurityFilter to not read request body

.. at least before the chain has been processed. Adds an integration test
to verify that the interaction between the filter and FormMapProvider
works as expected.
Adds failing test in BrooklynNodeRestTest to WIP group
Tidies getUser functionality in jsgui and quietens jsgui build
Miscellaneous tidies
Adds integration test for ServerResource.getUser
This closes #98
This closes #82
Fixes logic for potential seed count / quorumSize