Migrate NETCONF junos_helper to provider.

[aaronchar]

The purpose of this pull request is to migrate the junos_helpers functionality from go-netconf to this project, it is still using the go-netconf library but instead of depending upon the junos_helpers from the library we simply implement it within this repo.

This should allow for easier changes to the NETCONF calls that are being used from this provider while preventing any unwanted changes to the underlying libraries helper such as adding in a bulk/batch mode etc.

To accomplish this I have made the following notable changes.

• The terraform_providers directory now contains a netconf directory, this is essentially a like for like copy of the junos_helpers file from go-netconf with the exception that we now return an interface structure, the reason for this is so that we can add or remove more helper files as long as it satisfies the interface requirements.

• The config.go library has been moved from the terraform_providers directory and is now a generated resource, the purpose for this is so that we can correctly import and use any subdirectories within the terraform_providers directory eg: the netconf directory

• The function which copies files from the terraform_providers has been updated to also copy files within subdirectories while maintaining the proper pathing, I also added in some logging to the terminal around the files that have been copied over.

• when generating the provider.go file it now has the pathing to the local netconf library added in.

[aaronchar]

I know from comments made by @davedotdev that the idea is to keep this jtaf library as simple as possible,I think the added flexibility that is gained by moving a core dependency from an external library into the provider library allows greater flexibility as well as potential customization at a user level.

Right now, you are modifying multiple libraries if you want to implement a solution to #31 . This PR will make it so that we only need to alter the single jtaf NETCONF interface.

[aaronchar]

This has been updated to work with the latest sdk changes.

[aaronchar]

The latest changes to the PR clean up the NETCONF helper functions.

I have removed functions that are not needed for the operation of jtaf and I have condensed the destroy operations to a single function instead of destroyNoCommit and destory, this is now a single destroy function that takes a bool commit variables.

[aaronchar]

@ydnath - Just tagging you on this for a review.