Add environment attribute set/unset

[jeunito]

Sets/Unsets the value of a key in multiple environments in one call. Nested attributes are separated by a :

environment attribute set

$ knife spork environment attribute set dev hello:world chef
Modifying environment1
Done modifying environment1 at 2018-05-23 17:33:46 -0700
Modifying environment2
Done modifying environment2 at 2018-05-23 17:33:47 -0700

This creates the following in default_attributes section of the environment1 and environment2 json files:

"hello" : {
  "world": "chef"
}

environment attribute unset

knife spork environment attribute unset does the opposite:

$ knife spork environment attribute unset dev hello
Modifying environment1
Done modifying environment1 at 2018-05-29 17:30:21 -0700
Modifying environment2
Done modifying environment2 at 2018-05-29 17:30:23 -0700