Make mysql bind-address configurable

Make mysql bind-address configurable

It used to be hardcoded that the bind-address was always coming from
the $::hostname fact. This is wrong, as it disregards where we have
configured the mysql address. This commit actually makes it
configurable, so we'll be able to set it via hieradata.

On the other hand, we use the hiera key that we already set
'mysql_bind_host' as a default; if, for some reason, that's
unavailable then we fall back to $::hostname.

Related-Bug: #1627060
Change-Id: I316acfd514aac63b84890e20283c4ca611ccde8b