Fix Devstack docs for zsh users
Fix Devstack docs for zsh users

The current docs fail with the following error:
$ image=$(nova image-list | egrep "$DEFAULT_IMAGE_NAME[^-]" | awk '{print $2 }')
zsh: bad math expression: operand expected at `^-'

With this change, these docs work as expected for zsh users.

Change-Id: I56c39718cfe40208f62ea4786d684f6b24431ed9

naototty/vagrant-lxc-ironic,devananda/ironic,supermari0/ironic,debayanray/ironic_backup,dims/ironic,dims/ironic,ramineni/myironic,froyobin/ironic,redhat-openstack/ironic,bacaldwell/ironic,SauloAislan/ironic,faizan-barmawer/openstack_ironic,bacaldwell/ironic,pshchelo/ironic,rdo-management/ironic,Tehsmash/ironic,naototty/vagrant-lxc-ironic,pshchelo/ironic,ionutbalutoiu/ironic,rackerlabs/ironic,Tan0/ironic,NaohiroTamura/ironic,openstack/ironic,hpproliant/ironic,openstack/ironic,SauloAislan/ironic,ionutbalutoiu/ironic,NaohiroTamura/ironic,naterh/ironic