Summary:
I have updated the Vagrantfile to have an entry for CentOS 7. Also created a simple build script which is pretty similar to the one in Beringei.
How to test:
```
vagrant up centos7
```
Todo:
Implement -j X for the build.
Closes https://github.com/facebook/rocksdb/pull/3530
Differential Revision: D7090739
Pulled By: ajkr
fbshipit-source-id: 9f9eda5b507568993543d08de7ce168dfc12282e
