Make all packages architecture-independent

cloud-installer's binary packages are identical across architectures except for the Architecture field, so there's no reason for them to be architecture-dependent.

(For somewhat arcane reasons, these packages being "Architecture: any" blocks the promotion of the new version of syslinux to utopic, so I'd appreciate this being uploaded to utopic even if the other changes in master are going to take a while.  This is because installability checks are applied a bit differently for architecture-dependent and architecture-independent packages, and the new version of syslinux restricts syslinux-common to just amd64 and i386, which has some fallout further up the stack; making these binaries Architecture: all works around this, and is the right thing to do anyway.)