introduce env var GRIPHOME for instance_path customization
introduce env var GRIPHOME for instance_path customization

Use the environment variable GRIPHOME as instance_path if set, and fall
back to the default ~/.grip otherwise.

jbarreras/grip,mgoddard-pivotal/grip,ssundarraj/grip,joeyespo/grip,mgoddard-pivotal/grip,ssundarraj/grip,jbarreras/grip,joeyespo/grip