baremetal: Install iptables for Docker if enabled

If iptables is not installed, e.g. in the CentOS 8 cloud image, and
Docker iptables management is enabled, we get the following errors:

Failed to find iptables: exec: \"iptables\": executable file not found
in $PATH failed to start daemon: Error initializing network controller:
error obtaining controller instance: failed to create NAT chain DOCKER:
Iptables not found

This change installs the iptables package Docker iptables management is
enabled.

Change-Id: I3ba5318debccafb28c3cbce8e4e9813c28b086fc
Closes-Bug: #1899060