Set first node added to cluster as a seed

`ccm create test -b -i 127.0.1. -v 2.1.3Current cluster is now: test
01:30 PM:~/cstar/ccm[develop*]$ ccm add node1 -i 127.0.1.1 -j 7100 -b --binary-itf=127.0.1.1:9043
01:30 PM:~/cstar/ccm[develop*]$ ccm start`

This does not work because node1 is not set as a seed, so the cluster has no seeds. A workaround is to use `-s` in the add command, but ccm should be smart enough to know that this is the first node, and thus must be a seed.

The only caveat is if a user is planning to add multiple nodes in this way, but doesn't want the first one to be a seed.