Easyconfig: allow configs to be overridden with env vars

Based on the discussion in #700

It's possible to override an Easyconfig option with an environment
variable. The name should follow EB_EC_PARAM_<eb name>_<config>, for
example: EB_EC_PARAM_ZLIB_HOMEPAGE. Options can be strings, list and
dicts. Specify like:
EB_EC_PARAM_ZLIB_PATCHES="file1:file2"
EB_EC_PARAM_ZLIB_TOOLCHAINOPTS="pic=False:optarch=True"
EB_EC_PARAM_ZLIB_HOMEPAGE="www.somesite.com"

@boegel, @fgeorgatos please have a look. 

Currently, only EB specific overrides are possible. Something like EB_EC_PARAM_MAXPARALLEL would make sense but EB_EC_PARAM_NAME doesn't. Do we allow a limited set of general options? In principle, all options can be general, only name would give problems (I think).