Add interface for retrieving environment params

The goal of this commit is to add an interface to `metadata.Manager`
that allows retrieving of all environment params:
`getEnvironmentParams(envName)`.

This is done by:

1. Adding functionality to the snippet parser to get all environment
params out of the snippet read from
`environments/<env>/params.libsonnet`

2. Adding functionality to the snippet parser to get all component
params out of the snippet read from `components/params.libsonnet`

3. Having the `metadata.Manager` implementation handle the 'joining' of
params from 1) and 2), and returning that result.