GUACAMOLE-1290: Add SFTP support for public keys and correct SSH protocol issue.

[necouchman]

This pull request fixes up a couple of issues with public key imports for SSH/SFTP connections:

• Adds the required parameters so that public keys can be used for SFTP connections in addition to SSH connections.
• Moves the code that checks for the presence of the public key inside the block that checks for private key authentication. I'm still a little fuzzy on the purpose of importing a public key, but, if I understand the libssh2 functions correctly, there's no time that you'd want to try to import or use a public key if you weren't also authenticating with a private key. The code, as merged before, imported the public key regardless of whether a private key was present, and the if/else logic use also messed up the detection between public key and password-based authentication.

[mike-jumper]

LGTM - just some borked indentation.