Purge each basedir separately

This should close #64

Given a configuration like this:

    cachedir: '/srv/puppet/cache'
    sources:
      one:
        remote: '/srv/puppet/puppet.git/'
        basedir: '/srv/puppet/env_test'
        prefix: false
      two:
        remote: '/srv/puppet/puppet.git/'
        basedir: '/srv/puppet/env_test'
      three:
        remote: '/srv/puppet/puppet.git/'
        basedir: '/srv/puppet/env_test_more'

Output from "r10k deploy environment --verbose" is:

    [R10K::Task::Deployment::DeployEnvironments - INFO] Loading environments from all sources
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment three_testing
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment three_production
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment three_master
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment two_testing
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment two_production
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment two_master
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment testing
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment production
    [R10K::Task::Environment::Deploy - NOTICE] Deploying environment master
    [R10K::Task::Deployment::PurgeEnvironments - INFO] Purging stale environments from /srv/puppet/env_test
    [R10K::Task::Deployment::PurgeEnvironments - INFO] Purging stale environments from /srv/puppet/env_test_more

Result is:

    # ls /srv/puppet/env_test*
    /srv/puppet/env_test:
    master	production  testing  two_master  two_production  two_testing

    /srv/puppet/env_test_more:
    three_master  three_production	three_testing