ceph-mixin: fix ceph-mixin setup

ceph-mixin: fix ceph-mixin setup

Made following changes to files,

Makefile:
    Add needed 'tox' target to generate alert files
    Now we can do 'make generate' OR 'make test'
    to generate all the yaml files (and run tests)

alerts.jsonnet:
    Added an 'import' line to include 'config.libsonnet' file.
    This fix the errors in generating 'prometheus_alerts.yml' file

tox.ini:
    Added all the existing 'alerts-' targets to 'envlist'
    Added the missing 'alerts-test' target to 'testenv'
    Added 'jsonnet' to 'allowlist_externals', which prevents a
    deprecation waring
    A minor spell correction

lint-jsonnet.sh:
    Made errors more verbose.

Signed-off-by: Arun Kumar Mohan <amohan@redhat.com>