* Fix URL pruning when joining an etcd member

Problem:
Existing member clientURLs were checked if they contain the joining
node's IP. In some edge cases this would prune valid URLs when the
joining IP is a substring match of the only existing member's IP.
Because of this, it was impossible to e.g. join 10.0.0.2 to an existing
node that has an IP of 10.0.0.2X or 10.0.0.2XX:

level=fatal msg="starting kubernetes: preparing server: start managed database:
joining etcd cluster: etcdclient: no available endpoints"

Solution:
Fixed by properly parsing the URLs and comparing the IPs for equality
instead of substring match.
Signed-off-by: Malte Starostik <info@stellaware.de>
(cherry picked from commit b23955e8)
Signed-off-by: Brad Davidson <brad.davidson@rancher.com>