Add vc bonus support in rest api

[mzmssg]

Add vc bonus support:

bonus = unguaranteed resource = VC_MAX_CAPACITY- VC_CAPACITY

If configured, vc can use no more than VC_MAX_CAPACITY quota. Quota between VC_CAPACITY and VC_MAX_CAPACITY is borrowed from others, might be preempted. Quota under VC_CAPACITY is guaranteed.

Currently only implement rest api, will implement webportal in the future.

Step1: get admin token

curl -H "Content-Type: application/x-www-form-urlencoded" \
     -X POST http://restserver/api/v1/token \
     -d "username=YOUR_USERNAME" -d "password=YOUR_PASSWORD"

Step2: add or update vc

curl -H "Content-Type: application/x-www-form-urlencoded" \
     -H "Authorization: Bearer YOUR_ACCESS_TOKEN" \
     -X PUT http://restserver/api/v1/virtual-clusters/:vcName \
     -d "vcCapacity=CAPACITY" -d "vcMaxCapacity=MAX_CAPACITY"

For backward compatibility, if no maxCapacity provided, bonus token will be disabled

TODO:
webportal:

1. expose vcMaxCapacity box when add or edit VC.
2. show maxCapacity info in VC page.

[coveralls]

Coverage decreased (-0.2%) to 52.629% when pulling eaa49ab on zimiao/expose_vc_bonus into 055c78e on master.