[SRU Justification]
In xenial and earlier, mdadm declares a versioned dependency on initramfs-tools. This was an Ubuntu-specific dependency only needed for upgrades, and the version referenced is ancient (ca. 2008). In zesty and later, the dependency has been removed.
Since this is no longer relevant for upgrades and we would not otherwise have a dependency on initramfs-tools, we should drop this dependency to support building of images with initramfs-tools removed for systems that we know don't require an initramfs.
[Test case]
1. On a xenial default cloud image install, try to run 'sudo apt purge initramfs-tools'.
2. Verify that this tries to remove mdadm.
3. Cancel the removal.
4. Install mdadm from xenial-proposed.
5. Run 'sudo apt purge initramfs-tools' again.
6. Verify that mdadm is not removed.
7. Run 'sudo apt install --reinstall mdadm' and confirm that the package can be installed successfully without initramfs-tools installed.
[Regression Potential]
If a user needs an initramfs in order to mount their root device (whether or not it's on RAID), and mdadm is the only package on their system which depends on initramfs-tools, it is possible that the user may remove initramfs-tools and render their system unbootable.
This is unlikely because initramfs-tools is still part of the 'minimal' seed and is therefore a dependency of ubuntu-minimal; and it remains a dependency of the generic kernel image.
So long as initramfs-tools is installed, mdadm's initramfs hooks will continue to function as intended when required.
I accepted the package but the update bug part of the operation failed, anyway this is in -proposed now.