Use environment variables

Drop the handler_default_off entry point in favour of allowing the
default probability to be overridden through a "probability" environment
variable.  The previous "off" value is equivalent to a probability value
of "0" (or "0.0").

Also remove the regions.txt file in favour of a "regions" environment
variable containing a comma-separated list of region names, eg:
  ap-south-1,eu-west-1,us-east-1

Any whitespace around the names is ignored.

When the "regions" value is empty (the default), the region in which the
lambda was deployed is used instead.

Small cleanup of virtualenv requirements

Merge pull request #1 from bbc/restrict-logs-perm

Restrict CW logs permissions to bare minimum