Don't assume a Parameter value is a str

Currently show_stack and list_stacks will fail for templates
which have a Number in the Parameters.

This should be considered for backport to milestone_proposed.

Change-Id: I1b20babf0998f7714f8bc7bf144e03ff1d7c0c91
Fixes: bug #1160653
(cherry picked from commit b76eb46fdeb3431b5220dd909334ba64b7a2515b)