use_replica_index env variable to enable/disable replica index.
use_replica_index env variable to enable/disable replica index.

Replica index is disabled by default.

Change-Id: I8170b64dba9e4ea6f8f53c1e9f90e9f517727f57
Reviewed-on: http://review.couchbase.org/12746
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

mschoch/ns_server,t3rm1n4l/ns_server,membase/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,pavel-blagodov/ns_server,membase/ns_server,nimishzynga/ns_server,daverigby/ns_server,vzasade/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,daverigby/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,vzasade/ns_server,vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,daverigby/ns_server,mschoch/ns_server,nimishzynga/ns_server,daverigby/ns_server,mschoch/ns_server,nimishzynga/ns_server