Allow managing certain instance properties

[AlanCoding]

demo:

$ tower-cli instance modify 5 --enabled=true -v
*** DETAILS: Getting existing record. *****************************************
*** DETAILS: Getting the record. **********************************************
GET https://localhost:8043/api/v2/instances/5/
Params: []

*** DETAILS: Writing the record. **********************************************
PATCH https://localhost:8043/api/v2/instances/5/
Data: {'enabled': True}

Resource changed.
== ======== ======== ================= =================== ======= ================= 
id hostname capacity consumed_capacity capacity_adjustment enabled managed_by_policy 
== ======== ======== ================= =================== ======= ================= 
 5 awx_3          39                 0 1.00                   true              true
== ======== ======== ================= =================== ======= ================= 

This will require a corresponding AWX fix... linking shortly.

[coveralls]

Coverage decreased (-0.04%) to 64.409% when pulling 5ea8dcd on AlanCoding:instance_management into ca3f0b1 on ansible:master.

[coveralls]

Coverage decreased (-0.04%) to 64.409% when pulling 5ea8dcd on AlanCoding:instance_management into ca3f0b1 on ansible:master.