New method: modify_network_interface_attribute

Adding the modify_network_interface_attribute method on ec2 connection.
Allows users to modify various attributes of an ENI.

Addresses issue #1426
PEP8 and other style fixes to fit in with existing code.
Add a check to ensure a valid boolean, "true" or "false" are sent for boolean values before doing a call to the server.
Add modify_network_interface_attribute unit tests.
Test PEP8 line length fixes
Merge remote-tracking branch 'taylordt/modify-network-interface-attrib' into develop