Add some tests for os_update_readonly_flag

There is no test for os_update_readonly_flag api,
so add some for it.

Change-Id: Ice9a6ffea47dbaa492067f746edddcfbfc4eac4a
Merge "Add some tests for os_update_readonly_flag"
Rename create_test_server to create_temp_server

Change https://review.openstack.org/#/c/58645/ introduced bug 1247161 when
running under nose as the method name includes "test".
Renamed to not use test and updated HACKING to give advice against using
"test" in a method name.

Closes-bug: 1247161

Change-Id: I9f6d7d5fc9b3b94e1fbf395a62177c8df7e9f5b4