Allow overriding the RAPI port/scheme

This patch adds a very dumb way to override the port and scheme for
RAPI: if the master string doesn't contain a colon, we consider it a
simple hostname specification, and thus prepend ‘https://’ and append
‘:5080’, (the default RAPI port); otherwise, we consider it a fully
specified URL, and don't do any mangling to it.

This allows to use saved RAPI responses in testing, with the “file://”
scheme, or to use another port or non https mode with an actual RAPI
instance.