[BZ 1135629] add support for upgrading from RHQ 4.11 index

This commit adds a schema upgrade step for migrating from the metrics_index
table which was used up through RHQ 4.11. I have added some automated tests
with ReplaceIndexTest.java. Support for upgrading from RHQ 4.12 which
introduced the metrics_cache_index table will come in a subsequent commit.

I also added a skip attribute to the <step> element in the schema update files.
If a user is migrating from 4.9, 4.10, or 4.11, it wouldn't make a whole lot of
sense to migrate data from metrics_index to metrics_cache_index and then to
the new metrics_idx table. This functionality allows going directly from
metrics_index to metrics_idx.