Systems: Improve checking of user provided SSH keys.

[scblack321]

Look for ways to improve the user experience (and reduce support calls) in the area of registering ssh keys for a Tapis System.

One common problem is to have problems with the private key when it is converted from multiple lines to a single line for inclusion
into a json file. It appears that sometimes the text ends up with the character '' followed by 'n' when it should be a single embedded newline character. This may be happening during copy/paste.

There are probably other common problems we could check for in the provided data.

We might also consider having a utility for converting the ssh key files directly into a json file. Maybe have 1 or 2 endpoints that could be called to assist in creating a valid Tapis Systems credentials json file.

Also consider validating the keypair by using it to encrypt and decrypt some text.