Defer searching for opm-parser

The "opm-parser" module is expected to become dependent upon the ERT
library.  Defer searching for the parser module until we've
established whether or not ERT is availble.

Suggested by: @andlaus
Elucidate description string for opm-parser.

The 'opm-parser' module is (currently) designed to parse ECL-type
simulation models.  Reflect that fact in its description.
Merge pull request #505 from bska/prereqs-nit

Fix a few nits in "opm-core-prereqs.cmake"