fetch capacity and simulate task fitting

[lenhattan86]

CHANGES

1. fetch capacity report in details. for example:

   $ ./australis fetch capacity -s http://localhost:8081 --toJSON | jq .
   INFO[0000] Scheduler URL:  http://localhost:8081
   INFO[0000] gorealis clientConfig url: http://localhost:8081/api
   INFO[0000] Fetching capacity from  http://localhost:8081/offers
   {
   "non-dedicated": {
   "cpus": {
     "8": 2
   },
   "disk": {
     "54699": 2
   },
   "mem": {
     "6935": 2
   },
   "ports": {
     "1000": 2
   }
   },
   "vagrant/bar": {
   "cpus": {
     "8": 1
   },
   "disk": {
     "54699": 1
   },
   "mem": {
     "6935": 1
   },
   "ports": {
     "1000": 1
   }
   }
   }

2 simulate fitting tasks where we try to find at most how many tasks can fit at the moment. For example:

$ ./australis simulate fit -s http://localhost:8081 test/task_config.yaml
INFO[0000] Scheduler URL:  http://localhost:8081
INFO[0000] gorealis clientConfig url: http://localhost:8081/api
INFO[0000] Compute how many tasks can be fit in the remaining cluster capacity
1

TEST DONE

• integration test.
• most of tests done at api level https://github.com/aurora-scheduler/gorealis/pull/18

NOTES

This PR depends on another PR on gorealis: https://github.com/aurora-scheduler/gorealis/pull/18