Move CommandFailed exception to tempest.exceptions

When the CommandFailed exception was added it was only being used by
the CLI tests. However, it is generally useful for anything that is
using subprocess to make external calls. This patch moves it to
tempest.exceptions to make using it simpler for non-cli tests to use
the exception.

Change-Id: Ibf5f1cbbb847d32976b54c4484acfc3c0e3b4f48
Merge "Move CommandFailed exception to tempest.exceptions"
Merge branch 'master' of https://github.com/openstack/tempest