Enable keystone token caching by OSC

SDK uses python keyring library to enable token caching. Normally this
is requiring a proper desktop (interactive) session, but there are some
backend plugins working in non-interactive mode. Store cache in an
unencrypted file on FS (this is not worse than storing passwords in
plaintext).

Change-Id: I42d698f15db5918443073fff8f27b926126d1d0f
3 files changed