initial implementation of couchdb node with the copy of config

no config replication yet

Change-Id: I934f5cd8e601cf1a830af9a96b060777748f1762
config replication from ns_server node to couchdb node

Change-Id: Iae84c24bdb0cdecefaf0566b9c1459ae0b759109
make single_bucket_sup a parameter in ns_bucket_worker_sup

so the same supervisors can be reused for couchdb node

Change-Id: I8a753e5ec8cee5cfab6c581ba710e7d80d718b02
handle ns_server node rename on ns_couchdb node

Change-Id: I0212a061f9d4522a300c3cd7120679dbe0a9c14b
do not use couch_config from ns_config_default

since there will be no couch_config on ns_server node, use env variable
with default value instead

Change-Id: If4bc21d0f4bc2fef55be83e36aa7e70587a0cff8
make sure that modules for child nodes are loaded in the same order

... as for ns_server node

Change-Id: Iec5f19c11046506d18063932e8c312a40b17e675