feat: Custom composer options with the `COMPOSER_FLAGS` environment variable

[aguingand]

Summary

Related to #237

Deploy command generally doesn't need composer's "require-dev" dependencies. We can prevent installing these packages by adding the --no-dev option to composer install. This PR allows to add composer options in the following environment variable:

COMPOSER_FLAGS = "--no-dev"

There are other install options that may be useful depending on projects.

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[kitop]

Thanks for the contribution, @aguingand Do you mind adding a test for this?

[aguingand]

@kitop done !