universal debug flag

**Describe the problem/challenge you have**
kapp controller is especially hard to debug when it starts calling out to other binaries, and occasionally you just want a detailed log from imgpkg or something.

**Describe the solution you'd like**
a single environment variable in the kapp-controller deployment would kick all tools into debug level output, enabling us to drink sweet diagnostic elixir from the torrential firehose of TMI.