Add terraform version controls

[jaywcarman]

Add globals to set minimum and latest Terraform versions.

TERRAFORM_VERSION_MINIMUM: The minimum required version. If there is an
existing binary that matches the minor version and is equal to or
greater than this value then a new binary will not be downloaded.

TERRAFORM_VERSION_LATEST: The version that will be downloaded if there
is no existing binary, or if the existing binary does not meet the
minimum version requirement.

fixes #58

[kavya498]

@jaywcarman , can we get test results of this?

[jaywcarman]

@kavya498 I reworked this PR to remove the packaging dependency. I tested and verified:

• If there is no existing terraform executable in the tmp dir then TERRAFORM_VERSION_LATEST will be downloaded and used.
• Any existing terraform executable that is >= TERRAFORM_VERSION_MINIMUM AND <= TERRAFORM_VERSION_LATEST will not be updated (so user's with the existing 0.12.20 won't see any change, unless we bumped up the tf version min).
• Any existing terraform executable that is < TERRAFORM_VERSION_MINIMUM OR > TERRAFORM_VERSION_LATEST will be replaced by TERRAFORM_VERSION_LATEST.