Iscsi manager

Handling issue #282  . Provided ISCSI CLI support and Added testcases for the same
Operations supported are as follows:
CLI modules are divided into iSCSI targets section and snapshots section.
iSCSI commands 
```bash
usage: sl iscsi [<command>] [<args>...] [options]

Manage, order, delete iSCSI targets

The available commands are:
  cancel    Cancel an existing iSCSI target
  create    Order and create an iSCSI target
  detail    Output details about an iSCSI
  list      List iSCSI targets on the account
```
Snapshot Commands 

```bash
usage: sl snapshot [<command>] [<args>...] [options]

Manage, order, delete iSCSI snapshots

The available commands are:
  cancel                Cancel an iSCSI snapshot
  create                Create a snapshot of given iSCSI volume
  create-space          Orders space for storing snapshots
  list                  List snpshots of given iSCSI
  restore-volume        Restores volume from existing snapshot
```