Set chunkSize when creating a new GCS backend

[usererror]

This PR sets the chunkSize on the GCS Backend in New and adds a small regression test to TestBackend.

Rationale

I was taking a look at the GCS physical backend code and noticed that chunkSize was being parsed and checked but never set on the actual backend in New. Looking through the commit history, it appears that chunkSize was originally a package level variable but was later changed to a parameter within the Backend but it was never wired up inside of New.

Tests

To verify I wasn't crazy, I ran the new test before changing the New func:

$ go test ./physical/gcs
2019-04-30T02:54:15.062-0400 [DEBUG] configuring backend
2019-04-30T02:54:15.062-0400 [DEBUG] configuration: bucket=vault-gcs-testacc-352928274214603302 chunk_size=8388608 ha_enabled=true max_parallel=0
2019-04-30T02:54:15.062-0400 [DEBUG] creating client
2019-04-30T02:54:15.062-0400 [DEBUG] configuring backend
2019-04-30T02:54:15.062-0400 [DEBUG] configuration: bucket=vault-gcs-testacc-352928274214603302 chunk_size=8388608 ha_enabled=true max_parallel=0
2019-04-30T02:54:15.062-0400 [DEBUG] creating client
2019-04-30T02:54:49.938-0400 [DEBUG] configuring backend
2019-04-30T02:54:49.938-0400 [DEBUG] configuration: bucket=vault-gcs-testacc-7058591571723470507 chunk_size=8388608 ha_enabled=false max_parallel=0
2019-04-30T02:54:49.938-0400 [DEBUG] creating client
--- FAIL: TestBackend (1.82s)
    gcs_test.go:68: expected chunkSize to be 8388608. got=0
FAIL
FAIL	github.com/usererror/vault/physical/gcs	36.771s

After setting the chunkSize in New the tests are all green:

$ go test ./physical/gcs
ok  	github.com/usererror/vault/physical/gcs	69.402s