Since Ubuntu 15.04 the file 'pxelinux.0' is provided by different
package ('pxelinux') which is not being installed by DevStack.
Extra problem is that this package does not exsit for earlier Ubuntu
versions at all.
More over, while in Trusty 'syslinux' package depends on
'syslinux-common', it only recommends it in Xenial, but we need it
installed as it provides *.c32 modules required for syslinux.
This patch adds 'syslinux-common' and 'pxelinux' to required deb packages
list when distro is 'xenial' or 'jessie' (as this packaging change has
its origin in Debian) by using the distro pinning in the package lists
as supported by DevStack.
Change-Id: I473fc4eb52f28a456c3f6888d5c7b25e8e08be26
Closes-Bug: #1638940
d21564eaba