Replaces -instance flag with CUTTLEFISH_INSTANCE environment variable

The instance number is used to calculate the default values of many
other command line flags, so having it in a flag is less than ideal.

The other adventage is that it's no longer necessary to pass
--instance and --domain flags to the other host processes, because the
environment variable is visible there automatically.

Bug: 79170615
Test: run locally
Change-Id: I6a1ccb2207dd7866e725664bc36f1a5e63ec8beb