Add console commands to inspect ensemble system

This pull-request adds commands to inspect the ensemble/consensus sub-system to `riak_kv_console` (which can be use by `riak-admin`).

riak_kv_console:ensemble_status([]) prints an overview of the consensus system, including an overview of all running ensembles.

riak_kv_console:ensemble_status([Label) prints details about a specific ensemble, with Label corresponding to the printed label for that ensemble in the overview output.

Examples of command output in various scenarios can be printed by running riak_kv_ensemble_console:print_variations().

Sibling pull-requests: basho/riak_ensemble#26, basho/riak#558, basho/riak_core#602

Addresses issues: basho/riak_ensemble#9

---

Here's some example output (the same as printed by `print_variations`) for the overview command.

Output when strong consistency/consensus is not enabled in `riak.conf / app.config`:

```
============================== Consensus System ===============================
Enabled:     false
Active:      false
Ring Ready:  true
Paranoia:    medium (AAE syncing required)
AAE enabled: false

Note: The consensus subsystem is not enabled.

================================== Ensembles ==================================
There are no active ensembles.
```

Output when consensus is enabled, but has not yet been activated because there are too few nodes in the cluster:

```
============================== Consensus System ===============================
Enabled:     true
Active:      false
Ring Ready:  true
Paranoia:    medium (AAE syncing required)
AAE enabled: false

Note: The consensus subsystem will not be activated until there are more
      than three nodes in this cluster.

================================== Ensembles ==================================
There are no active ensembles.
```

Output when both configured and active, but when AAE is disabled when paranoia mode is medium or higher:

```
============================== Consensus System ===============================
Enabled:     true
Active:      false
Ring Ready:  true
Paranoia:    medium (AAE syncing required)
AAE enabled: false

Warning: Paranoia level is medium, but AAE is not enabled. Ensembles will be
         unable to become trusted and will never reach quorum.

================================== Ensembles ==================================
There are no active ensembles.
```

Output during normal operation:

```
============================== Consensus System ===============================
Enabled:     true
Active:      true
Ring Ready:  true
Paranoia:    medium (AAE syncing required)
AAE enabled: true

================================== Ensembles ==================================
 Ensemble     Quorum        Nodes      Leader
-------------------------------------------------------------------------------
    1         2 / 3         2 / 3      dev1@127.0.0.1
    2         0 / 3         2 / 3      --
```

Output during a view change (eg. ensemble is transitioning between two separate memberships):

```
============================== Consensus System ===============================
Enabled:     true
Active:      true
Ring Ready:  true
Paranoia:    medium (AAE syncing required)
AAE enabled: true

================================== Ensembles ==================================
 Ensemble     Quorum        Nodes      Leader
-------------------------------------------------------------------------------
    1         2 / 3         2 / 3      dev1@127.0.0.1
              2 / 3         3 / 3  
    2         0 / 3         2 / 3      --
```

---

Here's some example output (the same as printed by `print_variations`) for the ensemble-specific command.

Output under normal conditions:

```
================================= Ensemble #1 =================================
Id:       test
Leader:   dev1@127.0.0.1

==================================== Peers ====================================
 Peer  Status     Trusted          Epoch         Node
-------------------------------------------------------------------------------
  1     leading     yes             1            dev1@127.0.0.1
  2    following    yes             1            dev2@127.0.0.1
  3    following    yes             1            dev3@127.0.0.1
```

Output when a node is offline:

```
================================= Ensemble #1 =================================
Id:       test
Leader:   dev1@127.0.0.1

==================================== Peers ====================================
 Peer  Status     Trusted          Epoch         Node
-------------------------------------------------------------------------------
  1     leading     yes             1            dev1@127.0.0.1
  2    (offline)    --              --           dev2@127.0.0.1
  3    following    yes             1            dev3@127.0.0.1
```

Output during a view change:

```
================================= Ensemble #1 =================================
Id:       test
Leader:   dev1@127.0.0.1

==================================== Peers ====================================
 Peer  Status     Trusted          Epoch         Node
-------------------------------------------------------------------------------
  1     leading     yes             1            dev1@127.0.0.1
  2    following    yes             1            dev2@127.0.0.1
  3    following    yes             1            dev3@127.0.0.1
-------------------------------------------------------------------------------
  1     leading     yes             1            dev1@127.0.0.1
  2    following    yes             1            dev2@127.0.0.1
  4    following    yes             1            dev4@127.0.0.1
```