If Satosa is installed under a path which is not the root of the
webserver (ie. "https://example.com/satosa"), then endpoint routing must
take the base path into consideration.

Some modules registered some of their endpoints with the base path
included, but other times the base path was omitted, thus it made the
routing fail. Now all endpoint registrations include the base path in
their endpoint map.

Additionally, DEBUG logging was configured for the tests so that the
debug logs are accessible during testing.

Rebased to current master. When composing the paths, use os.path.join
primarily, since it handles empty strings and duplicate separators
logically.
As long as we use the BASE_URL in the OpenID Connect frontend as an
issuer, it's not possible to create multiple provider discovery URLs.
Add documentation and a comment to explain this limitation.

Avoid messing README.md with an unwanted line break.

If Satosa is installed under a path which is not the root of the
webserver (ie. "https://example.com/satosa"), then endpoint routing must
take the base path into consideration.

Some modules registered some of their endpoints with the base path
included, but other times the base path was omitted, thus it made the
routing fail. Now all endpoint registrations include the base path in
their endpoint map.

Provide a simple implementation for joining path components, since we
don't want to add the separator for empty strings and when any of the
path components already have it.

Additionally, DEBUG logging was configured for the tests so that the
debug logs are accessible during testing.

Even though the OIDC provider configuration has an element for setting
the issuer, for some reason it was rewritten to BASE unconditionally,
but this has broken provider endpoint discovery when multiple OIDC
frontends were in use.

Setting an alternative issuer should not be an encouraged setup,
although provider discovery should work either way. The recommended
setting is to use the BASE as the issuer, and we can leverage the
agressive configuration value replacement logic, which rewrites all
occurences of <base_url> to the value of BASE. The unit test was
modified to guarantee this behaviour, though.

Add base_path and endpoint_basepath to backend and micro_services

Co-authored-by: Ivan Kanakarakis <ivan.kanak@gmail.com>