MINOR: cfgparse-global: add dedicated parser for *env keywords

MINOR: cfgparse-global: add dedicated parser for *env keywords

This commit prepares the config parser to support MODE_DISCOVERY and, thus,
refactored master-worker mode. The latter implies, that master process reads
only the 'DISCOVERY' tagged keywords from the global section and it must call
for this an appropriate keyword parser.

So, let's move the code, which parses *env keywords, from the global section
parser to its own keyword registered parser.