Do not use the env proxy by default

By using the proxy env by default. Otherwise, you can’t tell any check
scripts to don’t use proxy at all. The only way would be to empty the
proxy url by --proxyurl= ""

Now by calling the aliveness script:
./check_rabbitmq_aliveness -H localhost => we don’t use the env proxy
./check_rabbitmq_aliveness -H localhost —proxy => we use the env proxy