Add environment_variable_groups endpoint

 - only /v2/config/environment_variable_groups/running and /v2/config/environment_variable_groups/staging exist as endpoints, other groups are ignored
 - only supports GET and PUT, no delete operation (just PUT an empty hash), and no list operation (the only groups are runtime and staging)

[#76745400]

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>