The current method to rewrite the mirror URL in the ubuntu element will
only rewrite archive.ubuntu.com, which leaves the security URL alone.
This is probably not intended for people who are creating images behind
firewalls.
Change-Id: I54a867e0e9d5a7c94324a5c00ff1a83b4e6e4524
Add a script to ubuntu's pre-install.d that will change
archive.ubuntu.com to a mirror specified in $DIB_DISTRIBUTION_MIRROR.
Change-Id: Idfd5a40ec8a1912a5de5dcdcaf9795946b0b53b8