Artifact cleanup scripts use unsafe ops

[Marc]

Describe the bug
make clean_webapp raises the following when used:

find ./src/main/webapp -not -name 'jenkins_assets' -delete
find: -delete: ./src/main/webapp: relative path potentially not safe
make[1]: *** [clean_webapp] Error 1
make: *** [build_all] Error 2

This halts the make flow so it can't be part of a complex make command.

To Reproduce

• Use make clean_webapp

Expected behavior
The make command should complete smoothly.

[Marc]

Fix as of #23