salt.states.composer composer_home=None should unset $COMPOSER_HOME [FEATURE REQUEST]

[spinitron]

Composer has logic to choose the Composer home directory. The logic is defeated when $COMPOSER_HOME env var is set.

Both states in salt.states.composer have default composer_home=/root and it's not obvious how allow Composer to choose.

1. /root is an odd choice, security wise, and I suspect would not usually be recommended by Composer/PHP experts.

2. It would be convenient to have a way to unset the $COMPOSER_HOME env var so that I can allow Composer to choose the Composer home directory.

Describe the solution you'd like

1. Idk what to do about Problem 1 given the likely breakage of changing such a default.

2. I want both salt.states.composer states to unset $COMPOSER_HOME env var if composer_home=None and it isn't in env.

Alternatively, let's use cmd.run and deprecate these states.