Make volume_glance_metadata_create compat with DB2

Make volume_glance_metadata_create compat with DB2

As DB2 does not support SQL insert via raw number type if
table column data type is "CLOB" ("TEXT" in sqlalchemy), we need
to convert numerical values to string before insert operations.

This problem was discovered through the
test_volume_create_get_update_delete_from_image test case in
tempest.api.volume.test_volumes_get.  The test case fails with
a DBError without this change in place.

Partial-blueprint db2-database

Change-Id: I5b378cce5b639df23d7003548f49bdab1eecf6a0