Fix optional keyring support, add basic keyring tests

Commit 06d9437e8388b369546d760607f17cb5022750e9 made using a keyring
optional by adding a function to handle the imports. The problem is that
import needs to be available at a global level for it to be usable in
other parts of the code.

The function is replaced by a top-level try/except.

Fixing the imports isn't enough to get keyring support working because
in get_auth_ref_from_keyring() the wrong token was being used when
evaluating token expiration. It was using the token already in the
HTTPClient object and not the one returned from the keyring.

Some simple tests were added to prevent future regressions. These
tests will be skipped if the keyring or pickle packages are not
installed.

Change-Id: I1fe2c9e5cdf275df2047018368da2e4b3d2d6de2
Fixes: bug #1183072
Merge "Fix optional keyring support, add basic keyring tests"