Fix storage-engine-cache size issue

- Allow for empty values
- Allow if you don't specify the --storage-engine-cache-size

k8s/mongodb/container/docker_build_and_push.bash

@@ -1,5 +1,5 @@
 #!/bin/bash
 
-docker build -t bigchaindb/mongodb:3.3 .
+docker build -t bigchaindb/mongodb:3.4 .
 
-docker push bigchaindb/mongodb:3.3
+docker push bigchaindb/mongodb:3.4

k8s/mongodb/container/mongod_entrypoint.bash

@@ -9,6 +9,7 @@ MONGODB_CRL_FILE_PATH=""
 REPLICA_SET_NAME=""
 MONGODB_FQDN=""
 MONGODB_IP=""
+STORAGE_ENGINE_CACHE_SIZE=""
 
 while [[ $# -gt 1 ]]; do
   arg="$1"