Support "." in map keys

[jen20]

This pull request brings together various fixes for #2143 and #7130. The acceptance tests added by @sodre now pass:

make testacc TEST=./builtin/providers/triton TESTARGS="-run TestAccTritonMachine_metadata"
==> Checking that code complies with gofmt requirements...
/Users/James/Code/go/bin/stringer
go generate $(go list ./... | grep -v /terraform/vendor/)
2016/07/14 12:39:07 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/triton -v -run TestAccTritonMachine_metadata -timeout 120m
=== RUN   TestAccTritonMachine_metadata
--- PASS: TestAccTritonMachine_metadata (140.83s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/triton 140.843s

cc @phinze

[radeksimko]

Also fixes (hopefully) #3980

[phinze]

This LGTM - I'm going to follow it up with an extra unit test so we get coverage on this change on every commit.

[ghost]

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