Refactored cleaner and added cleanup/utils

Moved context/cleaner to its own package, called cleanup, under
benchmark.context and renamed it to cleanup.py. To improve cohesion,
splitted benchmark/utils.py into two, and all cleanup related functions
were moved to benchmark.context.cleanup.utils. Removed cleanup function
in cleanup.py and references to it, since it's obsolete. Finally updated
all references and unit tests accordingly to reflect the changes.

bp benchmark-context-cleanup-refactor
Change-Id: Iaffc4e35e21b734a7b4cb77ffaf810112054ebae
Merge "Refactored cleaner and added cleanup/utils"