Add missing growroot initscript and pkg-map entries for Gentoo.
growpart was added to Gentoo with [1]
Update the readme to reflect reality too (fedora added with
I5630dc638f85b1e80795826ef36a306632075460)
[1] https://packages.gentoo.org/packages/sys-fs/growpart
Closes-Bug: #1539273
Change-Id: I29056c7297489ec04f37757dbe33976901eceb49
Add systemd/fedora support to growroot element. This involves
installing the correct packages, shipping the systemd service file and
ensuring it is enabled.
Note the required growfs/resize packages for Ubuntu/Debian are
installed in other places. This is probably a bug in that path, but I
have not addressed that here.
I have tested this with a F23 build with all openstack-infra elements,
uploaded to RAX, and it boots and resizes the main file-system.
Change-Id: I5630dc638f85b1e80795826ef36a306632075460
