server_id: Change format to vnn:pid.task_id, pid.task_id or pid

server_id: Change format to vnn:pid.task_id, pid.task_id or pid

This helps ensure the string cannot be ambiguous, while also ensuring
that it remains simple in the non-cluster case.

The asymmetry of reading get_my_vnn() but writing based on
NONCLUSTER_VNN is acceptable because in the non-clustered case, they
are equal, and in the clustered case we will print the full string.

Andrew Bartlett