Use VENV_PIP_TARGET environment variable as a default target for pip if present

[vzhestkov]

What does this PR do?

Make using VENV_PIP_TARGET environment variable value as a default target for installin modules with pip.
In case of specifying target to pip.install or pip.installed the specified target will be used instead.
And the modules will be installed to the default target in case if VENV_PIP_TARGET not present.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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