Revert to 0.12 behavior for integration test venv reuse

[arcivanov]

Introduce the flag integrationtest_python_env_recreate (default to False) to
allow the user to control whether each integration test runs in a fresh
venv.

fixes #813

[coveralls]

Pull Request Test Coverage Report for Build 1306487551

• 6 of 6 (100.0%) changed or added relevant lines in 2 files are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.7%) to 81.079%

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Files with Coverage Reduction	New Missed Lines	%
src/main/python/pybuilder/python_env.py	2	70.86%
src/main/python/pybuilder/reactor.py	4	90.95%

Totals
Change from base Build 1299752691:	0.7%
Covered Lines:	5037
Relevant Lines:	6055

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💛 - Coveralls