factorize getFileContentFromSlave() into CompositeStepMixin

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
rename checkSlaveVersion to checkSlaveHasCommand

update doc and tests

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge pull request #1470 from tardyp/uploadFile

factorize getFileContentFromSlave() into CompositeStepMixin