fs_template._ensure_dirs: handle EEXIST (529120)

There was a race inside fs_template._ensure_dirs which could cause it to
raise EEXIST if a concurrent process created the directory after
os.path.exists returned False. Fix it by using the util.ensure_dirs
function, which already handles EEXIST.

X-Gentoo-Bug: 529120
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=529120
Acked-by: Alexander Berntsen <bernalex@gentoo.org>