Refactor image building and build type selection

This is a major refactoring of how images are built and how they are used and labelled:

• The base images now export a set of toolchains like gcc-7-14 or clang-6-libcpp-17. These are simply defined by sourceable files in /duneci/toolchains. There is also a default toolchain for each image.
• Based on these toolchains, there is a single opts file called dune.opts. This file imports the selected toolchain and offers two extension points:
  • Images further down the chain can put files in /duneci/cmake-flags. These files will be sourced by the opts file after the basic flags have been set up and can append / override those flags.
  • Jobs can specify a variable DUNECI_CMAKE_FLAGS, which will be appended to the end of the flags.
• The core images are now templated on both the base image and the toolchain. Based on those parameters, the system now builds a much larger set of core images for different combinations of distributions and toolchains. The old images are still available under their short names, but the alternative opts files are gone.