Added PerforceSCM retrieval method to PerforceTagAction

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>
Fix missing substitutions in PerforceSCM and PerforceTagNotifier.
All auxiliary methods in MacroStringHelper have been converted to private.

Signed-off-by: Oleg Nenashev <o.v.nenashev@gmail.com>