Add -S/--superuser option for witness database creation

Previously the witness database creation code was hard-coding the
username 'postgres' when accessing the previously initialised database.
However initdb was not passed any explicit username, meaning the
default database superuser name was the same as the user running
repmgr.

With this patch, a superuser user name (default: postgres) will
be passed to initdb.

Per report by eggyknap [1]

[1] https://github.com/2ndQuadrant/repmgr/issues/38