Having an executor within applier package creates
potential import cycling. This patch moves it to a separate
package which can be used to conveniently store all the executors
at one place.
Change-Id: I68d909489b691e4fb7129446ef9a3fb085f8683c
Signed-off-by: Ruslan Aliev <raliev@mirantis.com>
Relates-To: #432