[FLOC-1000] Centos omnibus package builder

Fixes https://clusterhq.atlassian.net/browse/FLOC-1000

To Test:

* Build the packages
```
./admin/build-package --distribution=centos-7 $PWD
...
clusterhq-python-flocker-0.3.2-1.413.g623f0b9.x86_64.rpm
clusterhq-flocker-cli-0.3.2-1.413.g623f0b9.noarch.rpm
clusterhq-flocker-node-0.3.2-1.413.g623f0b9.noarch.rpm
```

* Start a centos7 vagrant VM
```
mkdir ~/centos7
cd ~/centos7
vagrant init chef/centos-7.0
vagrant up
```

* Copy the generated RPMs to the shared vagrant dir
```
cp ~/projects/HybridLogic/flocker/clusterhq-*.rpm ~/centos7
```

* Log in to the VM
```
vagrant ssh
```

* Install the base package
```
cd /vagrant
sudo yum localinstall clusterhq-python-flocker-0.3.2-1.481.g4c09feb.x86_64.rpm
```

* Install zfs repo
```
sudo yum localinstall --nogpgcheck https://download.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-2.noarch.rpm
sudo yum localinstall --nogpgcheck http://archive.zfsonlinux.org/epel/zfs-release.el7.noarch.rpm
```

* Upgrade the kernel and reboot
```
sudo yum upgrade kernel
sudo shutdown -r now
```

* Install the kernel headers
```
sudo yum install kernel-devel
```

* Install the meta-package
```
sudo yum localinstall clusterhq-flocker-node-0.3.2-1.481.g4c09feb.noarch.rpm
```

This should now be able to install all the dependencies for flocker-node and install zfs + kernel modules.

CentOS seems to prefer CentOS over Centos.  It might be nice to say a little bit more here, too.  "expected" is a good word to avoid in a test method docstring.