Backend limit API calls

[robertglonek]

Replace the following backend APIs with a single API call:

// return slice of strings holding cluster names, or error
ClusterList() ([]string, error)
// accept cluster name, return slice of int holding node numbers or error
NodeListInCluster(name string) ([]int, error)
// returns a string slice containing IPs of given cluster name
GetClusterNodeIps(name string) ([]string, error)
// returns a map of [int]string for a given cluster, where int is node number and string is the IP of said node
GetNodeIpMap(name string, internalIPs bool) (map[int]string, error)

Also, review internal calls in backends to limit the calls there.