Enable etcd clustering when behind proxy

[hunchback]

In the scenario when we want to connect two analyzers as follows:

analyzer (client) ----> proxy ----> analyzer (server)

To support this the following enhancements are needed

(1) Analyzer (client) needs to be able to set per peer crenetials:

etcd:
  peers:
  - analyzer1:
       url: http://proxy.ibm.com:2379
       user: <user>
       password: <password>
       cookie:
         <key1>: <value1>
         <key2>: <value2>

(2) Analyzer (server) can set address(s) it advertises (when a client connects to it):

etcd:
  proxy: proxy.ibm.com

(3) Analyzer (server) can set both client and peer (default is client+1) ports

etcd:
  server: 0.0.0.0:2379
  peer: 0.0.0.0.2380