[FEATURE] Support for variables in routing default values (Revision 3)

Currently placeholders are only supported in ``name`` and
``uriPattern``.

This adds support for ``default`` values::
  -
    uriPattern: 'some/pattern'
    defaults:
      someDefault: '<somePlaceholder>'

Releases: master
Resolves: FLOW-76
Change-Id: I4bfc06d64837d5096734cf9ec0c2988f3180c78a
Revision-Id: f2bafbb2343a80c6cdacfbd09059a71efeb6c99b