Fix: tools: get stonith_admin -T option working again

Regression introduced in 2.0.3 by 3910b6fec

This reverts commit 247eb303df934944c0b72b162bb661cee6e0ed8b
("Refactor: tools: drop unnecessary string duplication in stonith_admin")
and fixes a regression introduced when stonith_admin was converted to use
GOption.

The -T option is intended to override the client name passed to the fencer API,
but the client name was set to the default (crm_system_name) after option
processing had already been done, so any value for -T was overwritten by the
default, and its memory was leaked.

This commit sets the default only if -T was not used.