Fixes file cache TypeError
Fixes file cache TypeError

The internal cache (common.filecache) uses dogpile.cache's dbm backend,
which uses the bsddb package from the standardlib (in py2 only). bsddb
however breaks with a TypeError if the cache key is a unicode string.

This change fixes this by using the ``key_mangler`` arg on cache
creation, which (according to the dogpile.cache docs) is to be used with
dbm/bsddb.

Change-Id: Ie4911e31567d773357da99af3ded0548ad0b7db1
Closes-Bug: #1492205

openstack/python-ironicclient,NaohiroTamura/python-ironicclient,openstack/python-ironicclient,NaohiroTamura/python-ironicclient