buildMirrors: Do not error out if nothing to do.
buildMirrors: Do not error out if nothing to do.

In GpMirrorListToBuild.buildMirrors() if there are no mirrors to
build, then return True, so that gprecoverseg returns a return code
of 0.

Signed-off-by: Shoaib Lari <ad045e028025f2970ed8506438710c2da1df61d5@pivotal.io>
Signed-off-by: Nadeem Ghani <6210ba2d096145a21f72d2cc5992a921fd4dd884@pivotal.io>

lisakowen/gpdb,janebeckman/gpdb,Chibin/gpdb,xinzweb/gpdb,greenplum-db/gpdb,edespino/gpdb,50wu/gpdb,edespino/gpdb,janebeckman/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,Chibin/gpdb,janebeckman/gpdb,Chibin/gpdb,greenplum-db/gpdb,xinzweb/gpdb,yuanzhao/gpdb,50wu/gpdb,janebeckman/gpdb,Chibin/gpdb,Chibin/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,lisakowen/gpdb,xinzweb/gpdb,yuanzhao/gpdb,adam8157/gpdb,greenplum-db/gpdb,janebeckman/gpdb,50wu/gpdb,Chibin/gpdb,lisakowen/gpdb,ashwinstar/gpdb,edespino/gpdb,janebeckman/gpdb,50wu/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,xinzweb/gpdb,Chibin/gpdb,janebeckman/gpdb,edespino/gpdb,janebeckman/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,adam8157/gpdb,edespino/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,Chibin/gpdb,50wu/gpdb,yuanzhao/gpdb,xinzweb/gpdb,Chibin/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,yuanzhao/gpdb,adam8157/gpdb,jmcatamney/gpdb,janebeckman/gpdb,50wu/gpdb,xinzweb/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,adam8157/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,edespino/gpdb,edespino/gpdb,50wu/gpdb,greenplum-db/gpdb,edespino/gpdb,Chibin/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,janebeckman/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,edespino/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,edespino/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb