Source default config files in env.sh

See description of the bug in the below linked issue.  This change moves the sourcing of configuration options into the env.sh now that we changed from `sudo` to `su` and lost our environment settings.

Fixes: https://github.com/basho/riak/issues/563

Tested on Ubuntu 14.04 and tested a devrel to ensure it doesn't affect it.


## Test Run

```
(Reading database ... 60884 files and directories currently installed.)
Preparing to unpack riak-ee_2.0.0beta7-369ad9b8-1_amd64.deb ...
Unpacking riak-ee (2.0.0beta7-369ad9b8-1) ...
Setting up riak-ee (2.0.0beta7-369ad9b8-1) ...
Adding group `riak' (GID 113) ...
Done.
Adding system user `riak' (UID 108) ...
Adding new user `riak' (UID 108) with group `riak' ...
Not creating home directory `/var/lib/riak'.
Processing triggers for ureadahead (0.100.0-16) ...
Processing triggers for man-db (2.6.7.1-1) ...
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ riak ping
You need to be root or use sudo to run this command
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak ping
Node 'riak@127.0.0.1' not responding to pings.
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak start
!!!!
!!!! WARNING: ulimit -n is 1024; 65536 is the recommended minimum.
!!!!
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak ping
pong
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ cat /etc/default/riak
cat: /etc/default/riak: No such file or directory
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak stop
ok
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo vi /etc/default/riak
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ cat /etc/default/riak
export WAIT_FOR_ERLANG=1
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak ping
Node 'riak@127.0.0.1' not responding to pings.
vagrant@vagrant-ubuntu-trusty-64:/vagrant$ sudo riak start
!!!!
!!!! WARNING: ulimit -n is 1024; 65536 is the recommended minimum.
!!!!
riak failed to start within 1 seconds,
see the output of 'riak console' for more information.
If you want to wait longer, set the environment variable
WAIT_FOR_ERLANG to the number of seconds to wait.
```