add getVersion and listPlugin commands and update replicate command

The replication command string changed after replication became a plugin in gerrit.
This patch adds a getVersion[1] and a listPlugins[2] command to gerritlib.  The listPlugin
command is used to determine the correct replication string to execute in the replication
command.

[1] Only available on gerrit version >= 2.6
[2] Only available on gerrit version >= 2.5
Closes-Bug: #1273891
Partial-Bug: #1082781

Change-Id: I966037851b3f27536f415230edd1ae4e710321fb
Merge "add getVersion and listPlugin commands and update replicate command"