Fix VhdImage and ex_list_public_ips in ARM driver

[tonybaloney]

I noticed that AzureVhdImage __repr__ would never work because it doesn't have a location field.

Also, this string would never render.
https://github.com/apache/libcloud/blame/trunk/libcloud/compute/drivers/azure_arm.py#L1040-L1055

What is the correct URL? @techhat @tetron do you know?

[tetron]

If it's consistent with the other API endpoints then it probably should be:

        action = "/subscriptions/%s/resourceGroups/%s/providers/Microsoft.Network" \
                 "/publicIPAddresses" % (self.subscription_id, resource_group)

[tetron]

Reference: https://docs.microsoft.com/en-us/rest/api/network/publicipaddresses

[coveralls]

Changes Unknown when pulling 69a80e5 on tonybaloney:azure_arm_fixes into ** on apache:trunk**.

[tonybaloney]

merging