Add stricter func name validation

[peterj]

The valid function names contain a combination of lowercase letters, numbers and separators (., -, _). Name can only start and end with a letter or a number and there can only be one separator in the row (e.g. my---funcname is not valid, however my-func_name is valid).

Fixes #503

[peterj]

Thanks for the explanation! I figured there would be multiple places for this :)

I am thinking perhaps fixing the b) as part of this PR as well and have a separate issue for a) and unifying it.

How does that sound @rdallman ?

[rdallman]

sounds good, thanks. I should've split a) into two parts, sorry - I think here we would be well served to validate the image name (ie after constructing from name) across init, build, bump, push and deploy