Set UTF-8 encoding on process.pipe as well for Py 2.6 compat

Previously, we only explicitly returned unicode-encoded output for
process.check_output. Do the same for process.pipe so lsblk on
the diagnostic page doesn't crash when it sees UTF-8 characters.

Change-Id: I0801bcd989da89749272d1e0f16dd4d17129c97e
Signed-off-by: Ryan Barry <rbarry@redhat.com>