Implement node daemon conectivity tests

This patch adds in gnt-cluster verify checks for inter-node tcp
communication checks on the node daemon port for both the primary and
(if defined) secondary networks.

The output looks like (4-node cluster, one with the secondary interface
down):
* Verifying node node1.example.com
  - ERROR: tcp communication with node 'node3.example.com': failure using the secondary interface(s)
* Verifying node node2.example.com
  - ERROR: tcp communication with node 'node3.example.com': failure using the secondary interface(s)
* Verifying node node3.example.com
  - ERROR: tcp communication with node 'node1.example.com': failure using the secondary interface(s)
  - ERROR: tcp communication with node 'node2.example.com': failure using the secondary interface(s)
  - ERROR: tcp communication with node 'node4.example.com': failure using the secondary interface(s)
* Verifying node node4.example.com
  - ERROR: tcp communication with node 'node3.example.com': failure using the secondary interface(s)

Reviewed-by: imsnah