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
```bash
export ROCCI_CLIENT_KEYSTONE_TENANT='my_tenant_here'
```

## Ruby
```ruby
ENV['ROCCI_CLIENT_KEYSTONE_TENANT'] = 'my_tenant_here'
```