[BUG] 'WARNING Dhcp client is not running' on RHEL 8.0

## Description

The version of NetworkManager installed on RHEL 8.0 use the internal dhcp client by default:
```
NetworkManager[831]: [1570363540.7012] dhcp-init: Using DHCP client 'internal'
NetworkManager[827]: [1570413842.0257] dhcp-init: Using DHCP client 'internal'
NetworkManager[832]: [1571104032.7495] dhcp-init: Using DHCP client 'internal'
```

However **WALinuxAgent** is only checking for `dhclient`: https://github.com/Azure/WALinuxAgent/blob/e3ecde432c0fd6968bdb6d2774e316a61b3289d2/azurelinuxagent/common/osutil/redhat.py#L74-L75

Because of this we are getting this warning in *waagent.log*:

```
WARNING ExtHandler Dhcp client is not running.
WARNING ExtHandler Dhcp client is not running.
WARNING ExtHandler Dhcp client is not running.
```

## Version

```
# cat /etc/redhat-release
Red Hat Enterprise Linux release 8.0 (Ootpa)

# waagent --version
WALinuxAgent-2.2.32.2 running on redhat 8.0
Python: 3.6.8
Goal state agent: 2.2.42
```

Start with azurelinuxagent/common/osutil/redhat.py at the linked lines 74-75 and review how the agent checks for dhclient. Compare that check with the RHEL 8.0 logs showing NetworkManager's internal DHCP client, then verify that the resulting waagent.log no longer reports a false DHCP warning when the supported client is running.

Written by the indexing model from the issue text.