install-deps.sh: support INSTALL_EXTRA_PKGS

install-deps.sh: support INSTALL_EXTRA_PKGS

Instead of requiring other scripts to install packages independently,
teach install-deps.sh to install additional packages from the variable
INSTALL_EXTRA_PKGS. Now, other scripts should just set
INSTALL_EXTRA_PKGS and call install-deps.sh.

In particular, this fixes an issue installing packages in a clean (ex.
container) system that doesn't yet have repositories set up. Since this
task is performed by install-deps.sh already we avoid a chicken-and-egg
issue (or doing redundant work of setting up repos) in other scripts.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 8b529a0d416acd9b1fd102fd384ab0b64736e66c)