utils.execute: make it possible to set the environment variables
utils.execute: make it possible to set the environment variables

The custom environment variables for the process can be passed via the
`env_variables' keyword argument akin to oslo_concurrency.processutils.execute
While at it stop modifying the environment of the current process, alter
only the child processes' environment.

Change-Id: I78975caea920cf69b26a9cb6d01a75cd9ec15d37

openstack/bareon,openstack/bareon