Once process executes successfully, _run_command will return a tuple
consisting of three elements of process exit code and output to stdout
and stderr, which could be None if it does not intent to capture and
return them.

Original behavior is still available. As long as error is
raised from subprocess or process exits non-zero and stderr output is
not captured, rpkgError will be raised.

Several examples:

1. run rpmbuild to build SRPM and output to stdout and stderr are not
   captured.

   _run_command(['rpmbuild', '-bs', 'pkg.spec'])

2. run rpm to get package metadata from SPEC.

   _run_command( ['rpm', '-q', '--qf', '%{name}', '--specfile', 'pkg.spec'], return_stdout=True, return_text=True)

3. run rpm but error is raised.

   _run_command(['rpm', '-q', '-qf', '--specfile', 'pkg.spec'])

Signed-off-by: Chenxiong Qi cqi@redhat.com