Added isAlive() method to com.tek42.perforce.process.Executor

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>
Added optional timeout to com.tek42.perforce.parse.AbstractPerforceTemplate::getPerforceResponse()
Should prevent hanging of p4 checkout in case of https://issues.jenkins-ci.org/browse/JENKINS-15315

Signed-off-by: Oleg Nenashev <nenashev@synopsys.com>