hexdump is used in common function generate_hex_string which is
used by nova and heat. The current general dependencies do not
have this dependency covered, instead it is usually pulled in by
other implicit dependencies when a full devstack is built. In
cases where only a subset is built (like just Heat and keystone)
hexdump is missing.
Added unit tests for the generate_hex_string function.
Depends-On: Ib47d802a31a0f4c2a49daa7e6698e37c70a2365a
Change-Id: I77c8c2019fb8b8174cdfaed3e56ebf728f0732b7
Closes-Bug: #1558672
d99c29032b