## What does this PR do?
To test centos-7 on non centos systems add centos-7 image to the LXC suite.
## Why is this change important?
PR #2112 adds centos-7 support to the install scripts
## How to test this PR locally?
`sudo ./utils/lxc.sh build searx-centos7`
- add installation method for nginx sites, morty and filtron
- clean up PUBLIC_URL environment in and outside of containers
- clean up comand lines
- handle uWSGI quirks on fedora (emperor mode)
- handle Python quirks on debian (there is no 'python' command anymore)
- lib.sh: add die and die_caller functions
- lxc_suite_install_info is now a function
- lint: shellcheck
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
