utils/config:

Added capability for the user to set a "OQ_CONFIG_FILE" envvar, to specify
a custom config file to override defaults.
bin/openquake:

Added `--config-file` option, which sets the "OQ_CONFIG_FILE" envvar.
This allows the user to override select configurations with a custom config
file.
utils/config:

Added a `refresh` module level function, to reload configuration
on-demand.
bin/openquake:

Reload the configuration if the user specifies `--config-file`.
Merge pull request #1162 from larsbutler/config-file-option

--config-file option

[r=matley] [f=1174316]