An update to LP: #1235975 (permission fixing)

In LP: #1235975 I was a little too aggressive with the permission fixes, especially in light of LP: #1233521.

After discussion with jstrand, we will do the following:

* system-image itself will create its logfile with 0600, but it will not change the permission of an existing logfile, nor of the directory containing the log file.

* system-image itself will create the random /tmp subdir with the proper permissions (but indirectly, by way of tempfile.mkdtemp()'s normal operation).

The system-image packaging will do the following. Note that the paths here are contingent on the default paths in the client.ini file, so if someone changes those after installation, they are on their own.

/var/lib/system-image will be created by the packaging with the proper permissions. a postinst upgrade conditional can ensure the proper permission if the directory exists. Similarly with /var/log/system-image and /var/log/system-image/client.log. (/etc/system-image/client.ini is already handled by the packaging.)