Skip unconfigurable drivers for store initialization

All enabled/known store drivers are currently initialized and configured
by Glance when API daemon startup, and the driver(s) which could not be
configured correctly fail are not skipped. This patch explicitly skips
the stores with bad configuration and logs the same.

Change-Id: Ie14b1fb2f554e12cf82512f1d84390619d35a886
Signed-off-by: Zhi Yan Liu <zhiyanl@cn.ibm.com>
Merge "Skip unconfigurable drivers for store initialization"