add yum upgrade when doing online installation.
add yum upgrade when doing online installation.

otherwise it might meet:
http://mirror.premi.st/epel/7/x86_64/repodata/repomd.xml: [Errno -1] repomd.xml does not match metalink for epel

zstackio/zstack-utility,zstackio/zstack-utility,zstackorg/zstack-utility,mrwangxc/zstack-utility,mingjian2049/zstack-utility,live4thee/zstack-utility,zstackorg/zstack-utility,mrwangxc/zstack-utility,live4thee/zstack-utility,live4thee/zstack-utility,mingjian2049/zstack-utility,zstackio/zstack-utility,zstackorg/zstack-utility,mrwangxc/zstack-utility,mingjian2049/zstack-utility