configure.py: centralize the declaration of build modes
configure.py: centralize the declaration of build modes

Currently the declaration of build modes is scattered throughout the
script. We have several places where build modes are mentioned
hardcoded, and related configuration is also scattered in several data
structures. This commit centralized all this into a single data
structure, all other code uses this to iterate over modes and to mutate
their configuration.

This patch was motivated by the wish to make it easier to add a new
build mode, which is what the next patch does. This is not something we
do often, but I believe these changes also serve to make the code easier
to understand and modify.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla