Support meta-args for shell provisioner

[ggiamarchi]

Fix #205

To-do List

• Implement feature
• Unit tests

Summary

It adds the ability to use what i call a "meta-arg" with the shell provisioner. At this point, the only one supported meta-arg is ssh_ip. It corresponds to the IP address vagrant uses to SSH into the machine.

Usage example

config.vm.provision "shell", inline: 'echo "$1 : $2" > ~/provision', args: ['IP', '@@ssh_ip@@']

Technical consideration

• Initially, i wanted to use symbols to represent meta-args in the args attribute of the shell provisioner but args can only contain strings (restriction done by the shell provisioner itself). This is why i introduced the @@ marker the recognize meta-args.
• Curently the implementation is a bit specific to the ssh_ip meta-arg. It will change to be more generic if we add more meta-args.

/cc @kphonik @tacoben For comments, to confirm this match your requierements

[coveralls]

Coverage decreased (-0.94%) to 91.67% when pulling ec608bf on issue#205 into ef09748 on master.

[0x1b-xyz]

@ggiamarchi Looks good that should be enough for me to pull the value into the puppet session!

[artem-sidorenko]

@ggiamarchi @julienvey this PR probably breaks some other vagrant plugins which rely on provision hooks. See #248