Tenant id may be in UUID or Keystone ID format

This patch makes CLI accept tenant ID in either UUID or Keystone ID format. This patch fixes issue #19.

Tested this patch by successfully creating and deleting a router:
```
midonet> create router name 'new router'
router0
midonet> list router
router router1 name MidonetProviderRouter
router router0 name new router
router router2 name MidonetMetadataRouter
midonet> delete router router0
midonet> list router
router router1 name MidonetProviderRouter
router router2 name MidonetMetadataRouter
midonet> quit
```