Feature issue#12

Shards backed by foreign tables marked correctly in the metadata.

Tested with:

1. Postgres 9.4
    * foreign table - `SELECT`/`INSERT`
    * normal table (`RELKIND_RELATION`)  - `SELECT`/`INSERT`
2. Citus 3.0.1
    * foreign table - `SELECT` problematic - sent mail to @jasonmp85. But I don't think it is related to this fix, because when I test with undoing the fix, we still have the problem.
    * foreign table - `INSERT` OK
    * normal table (`RELKIND_RELATION`)  - `select`/`insert`

closes #12 
Also, move the declaration of `shardStorageType` here.