Add config option in core.yaml to switch state database from goleveldb to CouchDB

Added the following to core.yaml

    # stateDatabase - options are "goleveldb", "CouchDB"
    # goleveldb - default state database stored in goleveldb.
    # CouchDB - store state database in CouchDB
    stateDatabase: goleveldb
    couchDBConfig:
       couchDBAddress: 127.0.0.1:5984
       username:
       password:

Changes were also made to couchdb_txmgr to support the config option.

Changes were made to couchdb_test.go and marble_example.go to support the
config option in core.yaml and environment variables.

Marbles example can now be run with couchDb with the following command:

CORE_LEDGER_STATE_STATEDATABASE="CouchDB" go run marble_example.go

Adding a declaration for CORE_LEDGER_STATE_STATEDATABASE will work for peer,
couchdb_test and marbles example.

Change-Id: If02ac4f7c1c234d5f4c869875f3c331e35c9a066
Signed-off-by: Chris Elder <chris.elder@us.ibm.com>