Mesos-related updates to the 3.x version

[misterbisson]

Couchbase support for these Mesos-related tasks:

• [x] Pass Couchbase user/pass as an environment var
• [x] Pass service names as environment vars
• [x] Pass Consul location via environment var

Do these in 3.x, then integrate 4.x based on @tgross' work in https://github.com/misterbisson/triton-couchbase/pull/1

[misterbisson]

Fixes for this are in https://github.com/misterbisson/triton-couchbase/pull/6, with test case in https://github.com/misterbisson/clustered-couchbase-in-containers/pull/12

[misterbisson]

Releases:

• https://github.com/misterbisson/triton-couchbase/releases/tag/enterprise-3.0.3-r4
• https://github.com/misterbisson/clustered-couchbase-in-containers/releases/tag/1.2