Most consumers of util.ExecCommand() need to convert the returned []byte
format of stdout and/or stderr to string. By having util.ExecCommand()
return strings instead, the code gets a little simpler.
A few commands return JSON that needs to be parsed. These commands will
be replaced by go-ceph implementations later on. For now, convert the
strings back to []byte when needed.
Signed-off-by: Niels de Vos <ndevos@redhat.com>