Performance improvements on external resource loading

[allxiao]

Performance metrics shows the current two time killers are:

1. Mounting the PersistentVolumeClaim to the build pod
2. Fetching the builder image in a new node

For 1, kpack should provide other volume driver options such as host path, and/or allow user to bring their own PersistentVolume. For 2, should have some documentation on setting up some warm up daemons.

[chrono2002]

For 2, should have some documentation on setting up some warm up daemons.

1. Simply pre-exec image builder pod with cache (hostpath or better user specific) already mounted.
2. Then just fetch sources there from git on update.
3. Exec buildpack and pack the image and ONLY AFTER THAT save the cache.

Better: Do not stop builder pod when build is complete for the image. Instead let the user specify a builder TTL in the manifest to re-use already running builder for the image. That would kick ass!