Convert numeric values in instance metadata to strings

Prior to this patch, if the following was in a template as a
"OS::Nova::Server" resource property, the user would see a "BadRequest"
error from the remote API:

metadata:
  a: 1

Fix this issue by converting the value of each metadata entry to a
string before passing the metadata to the novaclient create call.

Closes-Bug: #1260476
Change-Id: I89d60620a3b852a7fcdd2f41135b1f12fbd428d8
Merge "Convert numeric values in instance metadata to strings"