Fix some docker test flakiness where image cleanup process may
contaminate other tests. A clean up process may attempt to delete an
image while it's used by another test.
* Making pull activity timeout configurable in Docker plugin config, first pass
* Fixing broken function call
* Fixing broken tests
* Fixing linter suggestion
* Adding documentation on new parameter in Docker plugin config
* Adding unit test
* Setting min value for pull_activity_timeout, making pull activity duration a private var