(PUP-2706) Change adduser to useradd in postinst

There has been an intermittent error where the installation of
puppet-common is failing on Ubtuntu 14.04. This error is
```
chage: the shadow password file is not present
/usr/bin/chage failed with return code 15, shadow not enabled, password aging cannot be set. Continuing.
chfn: PAM: Authentication failure
adduser: `/usr/bin/chfn -f Puppet configuration management daemon puppet' returned error code 1. Exiting.
```

Though `chage` is complaining about the fact that shadow is not enabled,
it looks like the installation is failing sure to the `adduser` call.
The thought is that `adduser` does some things under the hood that
require PAM to be configured in the build environment, which it isn't.
If we change the call from `adduser` to `useradd`, hopefully we will not
run into this issue. Note: it is difficult to test this due to the
intermittent nature of the issue.

Also, since we're removing the use of `adduser`, we no longer need to
ensure that package is available on the system.