clean up & speed up environment checks

[Flamefire]

os.environ is a dict hence the check for existance and access can usually be combined into a single access of the key avoiding the repeated lookup.
Especially for os.getenv which already returns None when the key doesn't exist.
Also use .pop(key, None) to remove a key without checking for it's existance first to avoid C&P errors

I initially noticed this (performance/code quality) issue for the get_software_*-functions:

    root = None
    if env_var in os.environ:
        root = os.getenv(env_var)

[Flamefire]

Merged develop again to rerun test.

[boegel]

lgtm