[gcp-vpc-move-vip] Add support for multiple alias IP ranges on one node

If a cluster contains two gcp-vpc-move-vip resources, only one can run
on a particular node at a given time. If a second gcp-vpc-move-vip
resource starts up on a node where one is already running, the existing
alias IP range is removed before the new one is added.

This places unnecessary limits on functionality. Per the GCP
documentation: "A VM instance virtual interface can have up to 10 alias
IP ranges assigned to it."

Configuring alias IP ranges
(https://cloud.google.com/vpc/docs/configure-alias-ip-ranges)

The existing behavior prevents one node from being able to effectively
host two floating IP addresses simultaneously (unless they are in a
contiguous range and can be managed as a single unit, which is
uncommon).

This patch modifies the RA so that it updates the existing list of
alias IP ranges attached to a VM, instead of re-initializing the list
to hold only OCF_RESKEY_alias_ip. With these improvements, multiple
gcp-vpc-move-vip resources can run on a single node, thus supporting as
many simultaneous alias assignments as GCP allows.