Make CDN-in-a-Box enroller use repo's TO client library

[zrhoffman]

What does this PR (Pull Request) do?

• This PR is not related to any Issue
• Updated the CDN-in-a-Box enroller to use the repo's TO client library v2 (instead of downloading the client library as a dependency)
• The directories that the enroller copies into the first build stage are now tracked in a dockerignore for easy updating

This fixes something I encountered when testing #4702, which changes the TO client library. The enroller was not using the repo's version of the client library:

#15 0.940 get "golang.org/x/net/publicsuffix": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at //golang.org/x/net/publicsuffix?go-get=1
#15 0.940 get "golang.org/x/net/publicsuffix": verifying non-authoritative meta tag
#15 0.969 golang.org/x/net (download)
#15 2.591 github.com/kelseyhightower/envconfig (download)
#15 3.408 get "golang.org/x/sys/unix": found meta tag get.metaImport{Prefix:"golang.org/x/sys", VCS:"git", RepoRoot:"https://go.googlesource.com/sys"} at //golang.org/x/sys/unix?go-get=1
#15 3.408 get "golang.org/x/sys/unix": verifying non-authoritative meta tag
#15 3.437 golang.org/x/sys (download)
#15 4.905 github.com/apache/trafficcontrol/vendor/github.com/lib/pq/oid
#15 4.905 golang.org/x/sys/internal/unsafeheader
#15 4.916 github.com/apache/trafficcontrol/vendor/github.com/influxdata/influxdb/pkg/escape
#15 4.917 github.com/apache/trafficcontrol/vendor/github.com/jmoiron/sqlx/reflectx
#15 4.920 golang.org/x/sys/unix
#15 4.920 github.com/apache/trafficcontrol/lib/go-log
#15 4.923 github.com/apache/trafficcontrol/vendor/github.com/go-ozzo/ozzo-validation
#15 4.924 github.com/kelseyhightower/envconfig
#15 4.932 github.com/apache/trafficcontrol/vendor/github.com/asaskevich/govalidator
#15 4.934 github.com/apache/trafficcontrol/vendor/github.com/lib/pq
#15 4.939 github.com/apache/trafficcontrol/vendor/github.com/influxdata/influxdb/models
#15 4.947 golang.org/x/net/publicsuffix
#15 4.961 github.com/apache/trafficcontrol/lib/go-util
#15 4.989 github.com/apache/trafficcontrol/vendor/github.com/jmoiron/sqlx
#15 5.050 github.com/apache/trafficcontrol/lib/go-rfc
#15 5.081 github.com/apache/trafficcontrol/vendor/github.com/influxdata/influxdb/client/v2
#15 5.167 github.com/apache/trafficcontrol/vendor/github.com/go-ozzo/ozzo-validation/is
#15 5.249 github.com/apache/trafficcontrol/lib/go-tc/tovalidate
#15 5.261 github.com/apache/trafficcontrol/vendor/gopkg.in/fsnotify.v1
#15 5.280 github.com/apache/trafficcontrol/lib/go-tc
#15 5.522 github.com/apache/trafficcontrol/traffic_ops/v2-client
#15 5.743 github.com/apache/trafficcontrol/infrastructure/cdn-in-a-box/enroller
#15 5.759 # github.com/apache/trafficcontrol/infrastructure/cdn-in-a-box/enroller
#15 5.759 ./enroller.go:256:9: assignment mismatch: 2 variables but toSession.Session.CreateDeliveryServiceServers returns 3 values

Which Traffic Control components are affected by this PR?

• CDN in a Box

What is the best way to verify this PR?

Build CDN-in-a-Box enroller image, verify that it enrolls servers

If this is a bug fix, what versions of Traffic Control are affected?

• master (9db1632)

The following criteria are ALL met by this PR

• CDN-in-a-Box is a test, no further tests included
• This PR does not include documentation because enroller behavior does not change
• This PR includes an update to CHANGELOG.md
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

#4700 fixes this too, closing as a duplicate fix.

[zrhoffman]

Re-opening because this issue affects the master branch now