support a specced property for yaml anchors

[rlindner81]

So, in our big project we have been using mta and mbt for long time. Some time this year a feature was added on the https://github.com/cloudfoundry/multiapps-controller repo, where mta's are validated for unsupported keys.

This validation consistently warns us now about a key that

...
parameters:
  defaults:  // <== this one
    - &bla
    - &blub

The resulting line that you see during cf deploy is:

Parameter(s) "{global-parameter=[defaults]}" are not supported in the specified scope, or referenced by any other entities. These parameters are not processed and will be lost after the operation completes. Further usage of these parameters might lead to unexpected results. Use at your own risk.

We want to put all our yaml anchors under a key at the beginning of the MTA and then use them throughout. There is currently no good key to do this, and I feel like there should be a place for this since anchors are really, really useful for serious projects.

Please consider adding a top-level key called, e.g. anchors that either mbt strips out when the yaml anchor resolution is done or everyone just ignores...