Removed duplicated class in exception.py
Removed duplicated class in exception.py

This commit 6f9ed562d5f6df1dc16029dcaffd4d2e6601099d had a mistake
which added two same classes ImageDownloadModuleMetaDataError to
exception.py, clear it now.

fixes bug 1223165
Change-Id: I2e7a30a63ab24d1ffb9f33a14db65c6fb3594cb0

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects