Change metastore file location from project to `/tmp`

After S2GRAPH-93, we use H2 as default metastore and test cases store metastore on ./var directory.

If I run `sbt test` then remove ./var directory then run `sbt test` again, then H2 metastore start their sequence from 1 again.

This is problematic since we are not actually delete data in HBase, but only deleting meta(`Label`, `Service` etc).

Deleting `./var` directory affect same `Label` id, which comes from auto increment on H2, and This make current test run conflict with previous test run.

To correct this, It would be better to drop data after each test cases finished on HBase, but that needs lots of changes on all test cases.

Following is options we can choose(please suggest better way).

1. manually truncate hbase tables before run `sbt test`.
2. relocate meta store directory for H2 from `./var` to `/tmp` so we don't have to rm `./var` directory when switch branches.

Please any feedback