Adds new method nova.utils.get_hash_str

Some modules are using their own method to generate hex from
string.
This patch adds a new method "get_hash_str" in nova.utils and
cleans every module needs to use it.

Change-Id: I2fa7dd2ea12c1d516224132d13454d109bb6c1e3
Merge "Adds new method nova.utils.get_hash_str"