ign to gz : release-tools and build scripts

[methylDragon]

See: #698

There is a lot to change in the release-tools repo, and it's complicated by the fact that there are multiple things to juggle that look similar, but aren't necessarily related or co-migrateable:

• CMake project names (e.g. ignition-gazebo7)
• CMake generated library targets (e.g. libignition-gazebo7-...)
• GitHub branch names (e.g. ign-gazebo6)
• Generated debian packages (e.g. libignition-cmake2-dev)
• Jenkins job names
• and more that I probably didn't manage to hit..

This makes it exceptionally difficult, since the similar looking names makes it hard to trace what refers to what, and there's also a possibility where names are shared/coupled by bash variables flying all around the scripts in this repo.

Also, it's ambiguous whether gazebo means Gazebo Classic, or Gazebo... (e.g. gazebo-base-default.bash)

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This is a tracking issue to keep track of (non-exhaustive) list of the things that can/need to be migrated, and where they occur:

• Links to github repos
• sed rules for bumping stuff in homebrew-simulation (bump_dependency.bash)
• DSL filenames (e.g. ignition_collection.dsl)
• jenkins script names (e.g. ign_cmake-default-devel-windows-amd64.bat)
• ABI_JOB_SOFTWARE_NAME, which takes information from variables in this file
  • It has conditional logic that appears in places like ignition-abichecker.bash that checks for equality with ign- prefixed names
  • This one is complicated because those names target branches, so naively changing gazebo to sim won't work
• ignition arguments and variables in release-bloom.py
• Dependency names in _generic_linux_compilation_build.sh.bash (e.g. IGN_CMAKE, which bubbles down to BUILD_IGN_XXX variables used elsewhere)

Because of how many items there are to migrate, let's split it up into several PRs:

• ign -> gz Partial Release Tools Migration #746 (This is a very rough PR that just tries to hit as many low hanging fruit as possible first)