RepoConfig: allow repos.conf 'masters' override

RepoConfig: allow repos.conf 'masters' override

This give the user the power to override 'masters' settings from
layout.conf, is case they want use a different set of masters to
satisfy dependencies. This is especially important now that recursive
expansion of masters is supported since commit
ab2a6cc357ba3c8272a4a1556e2c0bcd4bee102e, so that the user can avoid
pulling in unwanted repositories as dependencies, if necessary.

Also, empty 'masters' settings are now supported, in case a repo
wants to avoid implicit inheritance of PORTDIR settings like
package.mask.