Consider calling clean-salt prior to prep-salt targets

[emkll]

The clean-salt Makefile target [1] wll ensure that no old files are lingering in /srv/salt, in the event we remove a .sls file from the repo or from the RPM file without calling make clean. Adding it to prep-salt will ensure it's reliably applied when running make all or make $APP_VM. For RPM, whatever admin script we run can clean salt as part of the post step [3] or when applying the highstate (TBD, per #406).
[1] :

securedrop-workstation/Makefile

Lines 54 to 60 in 87b2c61

	clean-salt: assert-dom0 ## Purges SD Salt configuration from dom0
	@echo "Purging Salt config..."
	@sudo rm -rf /srv/salt/sd
	@sudo find /srv/salt -maxdepth 1 -type f -iname 'fpf*' -delete
	@sudo find /srv/salt -maxdepth 1 -type f -iname 'sd*' -delete
	@sudo find /srv/salt -maxdepth 1 -type f -iname 'securedrop*' -delete
	@sudo find /srv/salt/_tops -lname '/srv/salt/sd-*' -delete

[2] :

securedrop-workstation/Makefile

Lines 62 to 64 in 87b2c61

	prep-salt: assert-dom0 ## Configures Salt layout for SD workstation VMs
	@./scripts/prep-salt
	@./scripts/validate-config

[3]:

securedrop-workstation/rpm-build/SPECS/securedrop-workstation-dom0-config.spec

Line 73 in 87b2c61

%post

[zenmonkeykstop]

@rocodes to investigate.