CBG-738: Support use of cacert without certpath/keypath for DCP bootstrap

[sarathkumarsivan]

During DCP setup, SG/cbdatasource makes an initial bucket connection, then opens the DCP streams. The initial bucket connection only specifies the cacert if an x.509 cert is also specified (certpath/keypath). If a certpath isn't specified, the cacert isn't provided, and InsecureSkipVerify is set to true. However, the subsequent 'open stream' requests use the cacert and set InsecureSkipVerify=false, even if certpath/keypath isn't specified. The initial bootstrap should be fixed to use the same approach and change needs to be applied at the below three places:

1. While creating CBGT index definitions for the specified buckets
   createCBGTIndex: https://github.com/couchbase/sync_gateway/blob/master/base/dcp_sharded.go#L129

2. While starting the DCP feed
   StartDCPFeed: https://github.com/couchbase/sync_gateway/blob/master/base/dcp_receiver.go#L285
   StartCbgtCbdatasourceFeed: https://github.com/couchbase/sync_gateway/blob/master/base/dcp_dest.go#L446

[coveralls]

Coverage decreased (-0.02%) to 61.845% when pulling 023b521 on CBG-738 into 962ff96 on master.