Add proxy configuration for Replicator

In order for Replicators in different data centers to be able to communicate, the Replicator should be able to communicate with another Replicator through a proxy. 
For the Enterprise+ GA, both Artifactory and Replicator will use the configured default proxy in Artifactory for their part of the Distribution flows.

Requirements:

1. Each replicator should use the default proxy configured in Artifactory. (params will be passed during replicator registration)
2. If there is no default proxy configured, a proxy will not be used for the communication with the other replicator. 
3. If there is more than 1 default proxy configured, <TBD> 
4. Support Basic Authentication scheme by using Proxy http authentication/authrozation headers
5. support ssl proxy
6. communication between replicator and artifactory shouldn't be via proxy (different http client)