Filter fixed IP addresses with CIDR notation

Currently, etcd-manager returns the first found fixed IP address on VMs with multiple IP addresses in OpenStack.

However, there are multiple implications, for example:

• The first found IP address might not be the right one
• The on a VM IP address can vary everytime etcd-manager is executed due to inconsistent sorting of the results returned by the OpenStack compute API
• ETCD members can choose IP addresses located in different subnets which can lead to a non-functioning cluster

To mitigate this, let users specify a filter with CIDR notation so that instead of the first found fixed IP address the first IP address matching the CIDR notation will be returned.

When no filter is specified etcd-manager will still return the first found fixed IP address.