Fix vault paths, enable itest coverage, and display coveralls coverage badge

[aakselrod]

This PR updates the vault plugin paths to ensure that the node ID is in the path rather than in query parameters for the methods sign, ecdh, and accounts. This allows the node ID to be specified in granular ACLs to allow user assignment with various privileges to nodes.

For example, a user can have access to lnd-nodes/nodeApubkey but not lnd-nodes/nodeBpubkey. Alternatively, a user could have access to lnd-nodes/nodeApubkey/ecdh and lnd-nodes/nodeApubkey/accounts, but not lnd-nodes/nodeApubkey/sign, while having access to all of lnd-nodes/nodeBpubkey paths.

The PR also updates tests to work with the new path format, the README.md instructions, and adds an integration test to show that the node specified in the path isn't overridden by a query parameter specifying the node. This fixes #22.

The PR then updates lnd and bitcoind to the latest version, and vault to the version we use in our testnet sandbox.

Finally, the PR updates the integration tests to ensure lndsignerd and vault-plugin-lndsigner exit cleanly at the end of the tests, and enables coverage analysis on the integration tests. This fixes #21.

[aakselrod]

@jwelsh-nydig Can you take a look at this and let me know if this is what you want? I can't request you as a reviewer but would like your eyes on it.