Docker environment validation for docker-compose Update Module

Acceptance criteria:

• Add Docker validation to Update Module in MEN-3839:
  • Check that docker and docker-compose are available, and bail out if not.
  • If RootfsPartA is defined in mender.conf, which means that rootfs-image updates are enabled, then:
    • If /var/lib/docker is on the root filesystem, or points to a directory on the root filesystem, produce error and exit with a message saying that Docker storage needs to be on a different filesystem.