Add make container_stop target to makefile

[nitishm]

We now have,

• make container which builds the container,
• make container_run that depends on make container, and then runs the container, with name "vegeta".
• make container_clean that, currently removes the container and, removes the docker image.

Add a make container_stop target to the Makefile that would only do a docker rm -f vegeta.

Also update make container_clean to depend on make container_stop and remove the docker rm -vegeta step. make container_run should also, first depend on make container followed by make container_stop .

[nitishm]

@boydi0507 Do you want to take this on, since you just merged the docker changes. It should be quick.

[nitishm]

PS: Also store the container name "vegeta" in a Makefile variable, and use across the file.