Add a substring interpolation function

[joshuaspence]

Fixes #1431. Adds a new substr interpolation function which can be used to truncate a string.

[joshuaspence]

> make test TEST='./config' TESTARGS='-run=TestInterpolateFuncSubstr'
==> Checking that code complies with gofmt requirements...
==> Checking AWS provider for unchecked errors...
==> NOTE: at this time we only look for uncheck errors in the AWS package
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/03/22 08:50:59 Generated command/internal_plugin_list.go
go test -i ./config || exit 1
echo ./config | \
	xargs -t -n4 go test -run=TestInterpolateFuncSubstr -timeout=30s -parallel=4
go test -run=TestInterpolateFuncSubstr -timeout=30s -parallel=4 ./config 
ok  	github.com/hashicorp/terraform/config	0.011s

[grubernaut]

LGTM, thanks!

[dforsber-p]

substr: 'offset + length' cannot be larger than the length of the string

[grubernaut]

Hey @dforsber-p, I'm assuming this is an issue you encountered with the substring() interpolation function. Would you mind opening a detailed issue on Terraform with the problem you're seeing?
Thanks!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.