MINOR: vars: Parse optional conditions passed to the set-var converter

MINOR: vars: Parse optional conditions passed to the set-var converter

This patch adds the parsing of the optional condition parameters that
can be passed to the set-var converter. Those conditions will not be
taken into account yet in the var_set function so conditions passed as
parameters will not have any effect. This is true for any condition
apart from the "ifexists" one that is also used to replace the
VF_UPDATEONLY flag that was used to prevent proc scope variable creation
from a LUA module.