Add --pub-key= and --identity to gscloud-server-create

[bkircher]

gscloud-server-create creates a storage for you by default, --with-template allows you to select the corresponding template. The password is auto-generated for you.

Example:

$ gscloud server create \
   --name centos8 \
   --cores=1 --mem=1 \
   --with-template="CentOS 8" \
   --hostname centos8

Add three new flags to gscloud-server-create:

Each of those adds an SSH public key to the new root storage's authorized_keys file.

• --pub-key= takes a string argument as public SS key, uploads as new SSH key object, automatically derives a reasonable object name for it
• --identity= same thing, just accepts a path to a SSH key file or identity much as ssh(1)
• ssk-key=<id> uses an existing SSH key object already uploaded to gridscale