[SUREFIRE-823] Decouple -DskipTests from Failsafe plugin

[olamy]

The Failsafe plugin's skipTests parameter is no longer bound to the
skipTests user property. -DskipTests now only skips Surefire (unit tests)
while leaving Failsafe (integration tests) untouched.

Use -DskipITs to skip integration tests from the command line, or
-Dmaven.test.skip to skip both. The plugin configuration
element is still honoured when set explicitly.

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