Optional child token config

[nikolaishields]

This adds a new (often requested) vault provider configuration setting which can allow users to disable the creation of the intermediate child token (strongly discouraged, but made available for the cases that need it).

provider "vault" {
    address = "https://vault.testdomain"
    create_intermediate_child_token = false
}

(Open to changing the name of this config item, or related env var, but I like to be verbose with settings that typically shouldn't be adjusted without reading the documentation).

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Closes #29
Closes #722
Relates #550
Relates #368
Relates #192

Relates to the following as well:
hashicorp/terraform#16457
hashicorp/terraform#14839
https://groups.google.com/forum/#!topic/terraform-tool/wtlLrKVQlAo

Release note for CHANGELOG:

Add `create_intermediate_child_token` optional provider configuration setting to control the use of ephemeral child tokens for use by terraform.

Output from acceptance testing:

$ make testacc TESTARGS='-run TestAccChildToken'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccChildToken -timeout 120m
?   	github.com/terraform-providers/terraform-provider-vault	[no test files]
?   	github.com/terraform-providers/terraform-provider-vault/cmd/coverage	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/util	0.012s [no tests to run]
=== RUN   TestAccChildToken
--- PASS: TestAccChildToken (1.15s)
PASS
ok  	github.com/terraform-providers/terraform-provider-vault/vault	1.168s

[hashicorp-cla]

All committers have signed the CLA.