Fixes EGI-FCTF/rOCCI-api#11, Keystone tenants

Tenant selection for the Keystone authentication plugin can now
be controlled via the following ENV variable:

ROCCI_CLIENT_KEYSTONE_TENANT

Usage example:
==============

```bash
export ROCCI_CLIENT_KEYSTONE_TENANT='my_tenant_here'
```
```ruby
ENV['ROCCI_CLIENT_KEYSTONE_TENANT'] = 'my_tenant_here'
```
Merge pull request #16 from arax/keystone_tenant

Fixes EGI-FCTF/rOCCI-api#11, Keystone tenants