walinuxagent fails to start when host is using EC keys [BUG]

We're seeing the following error messages every 10 seconds on new Ubuntu hosts. Needless to say, none of the VM extensions are running, so we're unable to create new hosts.

```
2019/06/11 04:48:55.591593 INFO Daemon WireServer endpoint is not found. Rerun dhcp handler
2019/06/11 04:48:55.592240 INFO Daemon Test for route to 168.63.129.16
2019/06/11 04:48:55.594100 INFO Daemon Route to 168.63.129.16 exists
2019/06/11 04:48:55.595355 INFO Daemon Wire server endpoint:168.63.129.16
2019/06/11 04:48:55.609510 INFO Daemon Fabric preferred wire protocol version:2015-04-05
2019/06/11 04:48:55.610492 INFO Daemon Wire protocol version:2012-11-30
2019/06/11 04:48:55.612152 INFO Daemon Server preferred version:2015-04-05
2019/06/11 04:48:55.949508 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/1.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:55.962315 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/2.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:56.040907 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/1.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:56.053502 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/2.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:56.132301 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/1.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:56.140927 ERROR Daemon Command: [/usr/bin/openssl rsa -in /var/lib/waagent/2.prv -pubout 2>/dev/null], return code: [1], result: []
2019/06/11 04:48:56.169281 ERROR Daemon Exception processing goal state, giving up: ['']
2019/06/11 04:48:56.169792 INFO Daemon WireServer is not responding. Reset endpoint
2019/06/11 04:48:56.171328 INFO Daemon Protocol endpoint not found: WireProtocol, [ProtocolError] Exceeded max retry updating goal state
2019/06/11 04:48:56.186186 INFO Daemon Protocol endpoint not found: MetadataProtocol, [ProtocolError] 404 - GET: http://169.254.169.254/Microsoft.Compute/identity?api-version=2015-05-01-preview
2019/06/11 04:48:56.194682 INFO Daemon Retry detect protocols: retry=7
```

The change is that the new hosts have ECDSA certs in their cert store, configured via an ARM template.

The use of (non-walinuxagent related) elliptic curve certs appears relevant because the listed command fails:

```bash
root@vms0000000:~# /usr/bin/openssl rsa -in /var/lib/waagent/1.prv -pubout

140613926876824:error:0607907F:digital envelope routines:EVP_PKEY_get1_RSA:expecting an rsa key:p_lib.c:279:
```

but this command succeeds:
```bash
root@vms0000000:~# /usr/bin/openssl ec -in /var/lib/waagent/1.prv -pubout
read EC key
writing EC key
-----BEGIN PUBLIC KEY-----
MF..... (valid public key) ...==
-----END PUBLIC KEY-----
```

The elliptic curve certs are references in the VM Scaleset portion of the arm templates like so:
```json
{
"apiVersion": "2018-10-01",
"type": "Microsoft.Compute/virtualMachineScaleSets",
"name": "[variables('nt0ScaleSetName')]",
"location": "[variables('computeLocation')]",
"identity": {
"type": "SystemAssigned"
},
"properties": {
"upgradePolicy": {
"mode": "Automatic"
},
"virtualMachineProfile": {
...
"osProfile": {
...
"secrets": [
{
"sourceVault": {
"id": "[variables('keyvaultResourceId')]"
},
"vaultCertificates": [
{
"certificateUrl": "[parameters('cert1Url')]"
},
{
"certificateUrl": "[parameters('cert2Url')]"
}
]
}
],
...
},
```

Where cert1 and cert2 were previously RSA certs.

**Distro and WALinuxAgent details:**
- Ubuntu 16.04 (Azure images, plus some cloud-init and extensions in ARM templates)
- WALinuxAgent version:
```bash
root@vms0000000:~# waagent -version
WALinuxAgent-2.2.32.2 running on ubuntu 16.04
Python: 3.5.2
Goal state agent: 2.2.32.2
```

Note that this scenario should be completely supported, b/c using such ARM templates with Azure KeyVault references is a standard way of deploying certs to VMs, and KeyVault supports ECC certificates (though they still don't have support in the UI, they work via direct API access). Info here: https://docs.microsoft.com/en-us/azure/key-vault/about-keys-secrets-and-certificates