autopkgtest-buildvm-ubuntu-cloud: default to 2GB of RAM
We're hitting OOM issues with 512MB, e.g. when building ppc64el images.
2GB is what autopkgtest-virt-qemu uses by default; it makes sense to have autopkgtest-buildvm-ubuntu-cloud use the same. In constrained environments it is always possible to request less RAM via --ram-size.
LP: #1973628