Ignore docker service errors

Ignore docker service errors

The docker_service_enable switch was a hacky workaround to allow
molecule to be able to run the docker role. Rather than introducing
a variable that is not used anywhere an alternative is to just
ignore the error from the service enable command.

This command is expected to pass in VMs but fail in docker containers
due to missing systemctl. It is not critical that the service is
enabled so this is a reasonable alternative to using an otherwise
unused variable.

Change-Id: I25984ca2fd5e6141b29a23cb5feb4d1283eddd1a
Signed-off-by: Thanh Ha <zxiiro@gmail.com>