The related bug was caused because the charm did not detect when
the lxc storage had already been set up using the `lxc storage`
command which is used in the 3.x series, rather than the old way
with the 2.x series.
Also fixes issue on xenial where linux-image-extra-* has been renamed to
linux-modules-extra-* from from -4.4.0-145-generic onwards. Otherwise,
the install fails on xenial.
Change-Id: If8bdad6f5641ee21fc7860dceeb4c3facbaecc76
Closes-Bug: #1825393
4bf72620c3