Including tasks has a performance penalty when compared with importing
tasks. If the include has a condition associated with it, then the
overhead of the include may be lower than the overhead of skipping all
imported tasks. In the case of the check-containers.yml include, the
included file only has a single task, so the overhead of skipping this
task will not be greater than the overhead of the task import. It
therefore makes sense to switch to use import_tasks there.

Partially-Implements: blueprint performance-improvements

Change-Id: I65d911670649960708b9f6a4c110d1a7df1ad8f7