Add ability to grow XFS partition automatically in CentOS

One of the nice benefits of cloud-init is its ability to automatically grow an XFS filesystem upon first boot. This makes it so you can start with a small base image and deploy VMs with larger filesystems as needed.

The WALinuxAgent does not appear to support this, but these are the following steps I followed in order to grow the XFS file system of a CentOS image after growing its disk in Azure.

1. Install growpart script for growing a partition
```
yum install -y cloud-utils-growpart
```
2. Extend partition 1 in /dev/sda
```
growpart /dev/sda 1
```
3. Expand /dev/sda1 to fill entire partition
```
xfs_growfs /dev/sda1
```

Start by tracing WALinuxAgent's existing disk or filesystem resize handling and compare it with the reported CentOS steps using cloud-utils-growpart, growpart /dev/sda 1, and xfs_growfs /dev/sda1. Done means the agent can automatically grow an XFS filesystem after an Azure disk expansion on the stated CentOS scenario, with behavior verified against the existing agent checks.

Written by the indexing model from the issue text.