Added "skip" parameter to mojos

Because they take so long, I frequently need to skip integration tests
during my more frequent local builds. Given that this plugin is
primarily targeting the IT use case, it probably makes sense for these
executions o be similarly disabled.

Added "skip" to base mojos. Chose "skip", rather than enable/disable, to
correspond to skip parameters used to control surefire and failsafe
plugins.

Testing:

* Basic unit test added to StartContainerMojoTest
* Wired poms to leverage skipTests property to enab;e/disable docker
executions in the IT pom. Then ran regular maven install with mvn
install -P run-its -DskipTests=true
Added separate integration tests for skipping the plugin using a system property or plugin configuration. Added a test for AbstractDockerMojoTest for the skip option.
Updated documentation for the skip option.
Merge branch 'lcoote-master'