[requirements][heat] remove salt from requirements (used by heat-agents tests only)

Salt has been harsh to deal with. Upstream adding and maintaining caps has caused it to be held back. This time it's pyyaml, I'm not going to hold back the version of pyyaml for one import of salt. In any case, heat-agents uses salt in one location and may not even be using the one we define via constraints in any case. File: heat-config-salt/install.d/50-heat-config-hook-salt Installs salt from package then runs heat-config-salt/install.d/hook-salt.py In heat-config-salt/install.d/hook-salt.py is defined the only import of salt I can find and likely uses the package version as it's installed after tox sets things up. Is the heat team ok with this? -- Matthew Thode