Add centos7 image for rax

Rackspace provides a centos7 image, so add it to nodepool so we can
start testing with it.  I've tested the changes from
I80024d1afdb4e40d5fe9793ab2ec443b887c5fa8 manually in this image and
it is OK.

Change-Id: I2c040cc18d61b69a6f13cf0d8d102a5994c130e4
Merge "Add centos7 image for rax"