tighten up the grep expressions

The hive environment detection in the init scripts can sometimes blow up if multiple environment variables match the grep expression, for example ``HIVE_HOME`` and ``CDH_HIVE_HOME``.

This PR just tightens up the grep expressions

```
$ hive -e 'set -v' 2>/dev/null | grep '^env:HIVE_HOME='
env:HIVE_HOME=/opt/cloudera/parcels/CDH-5.2.1-1.cdh5.2.1.p0.12/bin/../lib/hive
```