Improve syntax for using multiple installations

The test automation framework allows installing multiple citus/postgres versions. The syntax is hard to follow:

```
fab set_pg_latest:/home/ec2-user/pg-960-citus-600
fab use.postgres:9.6.0 use.citus:v6.0.0 setup.basic_testing
fab set_pg_latest:/home/ec2-user/pg-961-citus-601
fab use.postgres:9.6.1 use.citus:v6.0.1 setup.basic_testing
# you now have 2 installations of Citus!
fab pg.stop # stop the existing Citus instance
fab set_pg_latest:/home/ec2-user/pg-960-citus-600 # switch to using the new instance
fab pg.start # start the new instance
# now you've switched back to the first installation

# the above can be abbreviated by writing the following:
fab pg.stop set_pg_latest:/home/ec2-user/pg-960-citus-600 pg.start

```

Also, there is no way of listing the installations via `fab`. An ideal solution would be somehow make it similar to `git branch`:

```
fab install:test_1 use.postgres:9.6.0 use.citus:v6.0.0 setup.basic_testing
fab install:test_2 use.postgres:9.6.0 use.citus:v6.1.0 setup.basic_testing

fab checkout: test2
fab list_installations
outputs: `(test_1, test_2)`
...
```