maven_jar: Make sha1 attribute optional
maven_jar: Make sha1 attribute optional

This will simplify to consume artifacts from local Maven repository.

Change-Id: Ie2b2b6103e16361b187202bddd8ecf5dae61f75f

qtproject/qtqa-gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,WANdisco/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit