[#49] Do not create a login shell in `elasticsearch.init` script

Do not use the '-' flag to `su` since it creates a login shell. Doing that
will prevent the environment from being inherited unless you also use '-m'.

Using '-' and '-m' when running bash will cause the launched shell to
try to read $HOME/.bash_profile, since $HOME still refers to the home
directory of root and the elasticsearch user does not have read access there.

A simple solution is to omit '-' and '-m'. The program will still run under
the `elasticsearch` user but will inherit environment variables and will not
print an error message.

Closes #49