clean configuration legacy for useProxy configuration

[wiiitek]

Currently there is a possibility to configure web proxy with "true" value.
This makes the code unclear: see also the discussion for pull request at:

• Removing not needed parameter #53 (comment)

For resolution I like the second approach more:

there will be only named type of a proxy (remove true as a possible value) after deprecating it first for e.g. 2 minor versions

(Alternative solution is to have the configuration value "default" be interpreted as "rest" proxy.)

Currently all suites using web proxy have it configured to "rest" as otherwise it doesn't work.

[malaskowski]

As currently true does not work, i think we can get rid of it with no deprecated period.