Add warning if both snap and deb lxd detected

[johnsca]

A somewhat common cause of issues with LXD and conjure-up is when both the deb and snap version are installed and the operator is using the deb version on the CLI while conjure-up prefers the snap. We should detect if both are installed and pop up a warning dialog making sure that the operator knows to use the snap version (check their PATH), or perhaps even to select which one to use.