Read in private key contents on connect

[worksofliam]

Changes

Fixes #1422.

Newer versions of node_ssh (and therefore ssh2) require us read the file and pass the buffer.

Checklist

• have tested my change
• updated relevant documentation
• Remove any/all console.logs I added
• eslint is not complaining
• have added myself to the contributors' list in CONTRIBUTING.md
• for feature PRs: PR only includes one feature enhancement.

[worksofliam]

To test this:

1. Use 1.9.4 release to connect with a private key and it should work.
2. With the master branch, try using the same connection with a private key on master and it should fail.
3. Try using this branch to connect and it should be successful.

[chrjorgensen]

@worksofliam I don't get any errors when running master branch and connection using a private key? It works as release 1.9.4?

[worksofliam]

@chrjorgensen can you confirm you also npm installed after switching between branches?

Edmund was getting this issue on master.

[worksofliam]

Ok, looking it further and it seems that ssh2 now only supports these formats: OpenSSH, RFC4716, or PPK format

@edmundreinhardt, can you npm i on master (not this PR) and see if the problem continues? Also ensure your key is one of the above formats.

[chrjorgensen]

@worksofliam Ah, I didn't do npm i before testing the master branch - so I've run on the old SSH2 extension. After doing npm i it fails as expected. Will test further...

[chrjorgensen]

@worksofliam Your PR works as expected and fixes the private key problem. Good work!