Added chef-vault as option to store secrets

*Added gem chef-vault to gemfile
*Added method to decrypt chef-vault items
*Added case for ['openstack']['databag_type'] = 'vault'
*Added version for ['openstack']['vault_gem_version'] = '~> 2.3'
*Moved ::Chef.log.warn for developer_mode from method 'secret' to private method
'dev_secret' to refactore and shrink the method 'secret'
*Added link to opscode chef-vault documentation

Change-Id: Icda0e288bbb920498aba856fefaf04c0f1852787
Merge "Added chef-vault as option to store secrets"