followups to build webhook trigger secret references

Follow ups to:
https://github.com/openshift/origin/pull/17314

• buildtriggercause includes the secret value partially masked. should not expose it at all: https://github.com/openshift/origin/pull/18389
• webhook trigger docs needs updating: https://github.com/openshift/openshift-docs/pull/6552
• quickstarts need to be updated to use the new reference mechanism
• new-app needs to be updated to produce webhook triggers that use secret refs
• do we care about the clutter of secrets that will be left behind when people delete their buildconfigs? should we use GC to clean them up?

Unfortunately changes to new-app and the quickstarts won't be backwards compatible, so we can't change new-app until 3.10. Not sure when we'd want to change the quickstarts.

/cc @jim-minter