Use execute() and trycmd() functions from common code

Update manila's functions 'execute' and 'trycmd' for
using same functions from oslo common code with
Manila's rootwrap.

Partially-implements blueprint use-common-code

Change-Id: I6d6a787d3d091a72dac635289b6647c00fb7df40
Merge "Use execute() and trycmd() functions from common code"