Install static element files

Several elements want to install static files. Some do this using the
"os-apply-config" directory, which means these files are unnecessarily
rewritten every time os-refresh-config triggers. Others put files in a
"files" dir and manually script the installation from an install.d
script.

This patch offers a more convenient "static" directory. Files placed in
here will be rsynced into the image. This assumes all files are meant to
be installed with user and group root.

Co-Authored-By: Gregory Haynes <greg@greghaynes.net>
Change-Id: I390b822bf80ff6470b25a3473326cac362c4497a