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
Merge "Fix Devstack docs for zsh users"