Expands remote instance client functionality

 * Makes linux (ssh) client agent aware, preventing paramiko
   from searching local ssh keys by default
 * LinuxClient
   * Better path concatenation in create_file method
   * Added get_all_disk_details method.
   * Changes get_md5sum_for_remote_file parameter signature to
     handle passing in paths and filename (so that tests don't
     have to worry about how the remote OS needs them joined)
   * Added generate_endpoint method (to match method in windows
     client)
   * Added get_disk_fs_type method
 * WindowsClient
   * Added unmount_disk method
   * Added get_all_disk_details method
   * Added generate_mountpoint method that returns the next
     available windows drive letter (for mounting disks)
   * Added get_disk_fs_type method.

Change-Id: Ib80416d624f8fa05fe39dded6f5099a8a0620d15