Add mount package for cloudinit+nocloud variant

Within a qemu VM running the cloudinit-nocloud variant (built from 759d4739), I was unable to mount a cloudinit iso such that its configuration would be read by cloudinit. I tried with a few different configurations, but consistently received an error failed to mount /dev/sr0 when looking for data. Reading through alpine's README for the cloud-init package, it appears this is a known error and caused by the lack of the mount package or a preconfigured fstab.

Adding the mount package for the cloudinit-nocloud variant was enough for cloud-init to detect and read the mounted iso in my environment.