fileutils is graduated in the oslo.utils library.
Remove old implementation of the read_cached_file function
in the nova.utils and move from the fileutils the read_cached_file
and delete_cached_file functions to the nova.utils module and
the unit tests for these functions.
Implements: blueprint graduate-fileutils[1]
[1] https://blueprints.launchpad.net/oslo-incubator/+spec/graduate-fileutils
Depends-On: I51ba9076e1fbc16145ee2311f47b7768c16dcb20 (requirements)
Change-Id: I849f1c74ec811dbe82ba6270569a008a49eab465
741f163fc5