Incorrect help of export_path for the manage utility

Incorrect help of export_path for the manage utility:

Steps to reproduce:
1. Execute 'manila help manage' command.
2. See <export_path> help description.

Expected result:
<export_path> Share export path, NFS share such as:
                      10.0.0.1:/foo_path, CIFS share such as:
                      \\10.0.0.1\foo_name_of_cifs_share

Actual result:
<export_path> Share export path, NFS share such as:
                      10.0.0.1:/foo_path, CIFS share such as: \10.0.0.1
                      oo_name_of_cifs_share

Fix proposition:
In python-manilaclient/manilaclient/v1/shell.py add more escape characters:
help='Share export path, NFS share such as: 10.0.0.1:/foo_path, '
          'CIFS share such as: \\\\10.0.0.1\\foo_name_of_cifs_share')