Use built-in DRBD module instead of building it manually in setup_nfs_ha.sh

Starting from the ubuntu-azure kernel version 4.15.0-1018-azure, the DRBD kernel module is now built in the default kernel modules package (linux-modules-4.15.0-1018-azure) thanks to Azure LIS team's effort, so we should stop building our own custom kernel module in `setup_nfs_ha.sh`, but just use the built-in module. Note that the built-in module's DRBD version is 8.9, whereas the custom build is with version 9.0. The `drbd-utils` package from the Ubuntu repo will be also about version 8.x, and there seem to be some incompatibility between the two version on the `drbdadm` command (e.g., no `drbdadm status` available with the `drbd-utils` package version), so related commands should be changed accordingly as well.