Fix metadata2html.py, add some more documentation to ccpp_prebuild.py

[climbfuji]

This PR fixes https://github.com/NCAR/ccpp-framework/issues/437, see description there, and provides an additional comment for one subroutine in ccpp_prebuild.py

User interface changes?: No

Fixes: #437

Testing:

• metadata2html.py works for in-source and out-of-source builds
• ufs-weather-model compiles and runs and gives b4b identical results

[climbfuji]

How easily things break.

Yes, especially when they are not tested/used regularly. capgen will do much better!