Added common code into fileutils and strutils.

This methods are present in nova, cinder and also needed by ironic package.

openstack.common.strutils => nova.utils <=> cinder.utils
    to_bytes(text, default=0)

openstack.common.fileutils => nova.utils <=> cinder.utils <=> nova/virt/libvirt/utils.py
    file_open(*args, **kwargs)

Change-Id: Ia65499b77eb551c0ffb6d48767b899435469bfb6
Implements: blueprint image-tools