fuel-agent: unicode handling in cloud-init templates

Apparently, fuel-agent uses `write-mime-multipart` [1] for aggregating multiple types of cloud-init user-data payloads into a signle one user-data file.

this tool doesn't have abitlity to specify non-default charset, thus it will be always producing 'us-ascii' plain text files. that wasn't an issue when all input data was limited to ASCII values (which is still purely compatible with UTF-8).

even if any of partial cloud-init payload contains non-ascii characters, it will use 'us-ascii' charset in the resulting multipart file due to [2]:

            # Note: we should handle calculating the charset
            msg = MIMEText(fp.read(), _subtype=subtype)

MIMEText supports specifying of the output charset [3]. However, `write-mime-multipart` isn't passing the charset at all.

[1] https://github.com/openstack/fuel-agent/blob/d40821e9a7b12a7ec6045c765c0e4b65f6df3239/fuel_agent/manager.py#L392-L396
[2] http://bazaar.launchpad.net/~cloud-utils-dev/cloud-utils/trunk/view/head:/bin/write-mime-multipart#L87
[3] https://docs.python.org/2/library/email.mime.html#email.mime.text.MIMEText