Enhance configuration handling

1. Changed drivers to prevent using global CONF but access it from
client by interface.
2. Corrected swift driver to involve options from swift driver utils,
allow glance_store exports and registers them for client.
3. Added missing test cases for the functions of swift driver util.

Change-Id: I8fac6800efde202e29179791ea05c4814ec58435
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Enhance configuration handling"