agent stops reporting instance-id on hp cloud compute

[xcolour]

When the agent first starts up, info reports:

  Hostnames
  =========

    ec2-hostname: dev-1
    local-ipv4: 10.5.135.198
    local-hostname: dev-1
    socket-hostname: dev-1
    public-hostname: dev-1
    hostname: dev-1.novalocal
    instance-id: i-0018ef9b
    public-ipv4: 15.185.125.129
    socket-fqdn: dev-1.novalocal

But then later it reports:

  Hostnames
  =========

    socket-hostname: dev-1
    local-ipv4: 10.5.135.198
    public-hostname: dev-1
    hostname: dev-1.novalocal
    public-ipv4: 15.185.125.129
    socket-fqdn: dev-1.novalocal

The metadata service is still available, though:

$ echo $(curl -s http://169.254.169.254/1.0/meta-data/instance-id)
i-0018ef9b

[xcolour]

Looks like the metadata service times out occasionally. When it times out, we treat it as if the metadata doesn't exist. Instead, we should stick with the last good value.