add btrfs-tools and xfsprogs to seeds

[Scott Moser]

xfsprogs and btrfs-tools enable creating and working with filesystems.
They're useful to have in the cloud image so that cloud-init or other machinery such as curtin can create filesystems to work with.

The reason for wanting them in xenial cloud image is so that they get into xenial maas image and then curtin can use them there.
It does seem to make reasonable sense to have supported filesystem tools available in our images.

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

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[Scott Moser (smoser)]

this is present in xenial and later.