Fix argument error for --platform option

[dheerajd-msys]

Signed-off-by: dheerajd-msys dheeraj.dubey@msystechnologies.com

Description

• knife ec2 ami list --platform returns ERROR: ArgumentError: unexpected value at params["filters"][0][:platform]
• Handles the error by passing appropriate content to filter array

Issues Resolved

Fixes #607

Output Post Fix

PS E:\Backup\Project\Chef_Repo\knife-ec2> bundle exec knife ec2 ami list --platform windows -c 'E:\Backup\Project\chef-starter\chef-repo\.chef\knife.rb' --owner microsoft
AMI ID        Platform  Architecture  Size  Name            Description
ami-c4169af4  windows   x86_64        35    Windows_Server
ami-c6169af6  windows   x86_64        35    Windows_Server
ami-c8169af8  windows   x86_64        30    Windows_Server
ami-c827b8f8  windows   x86_64        30    AWS             aws dc

Check List

• New functionality includes tests
• All tests pass
• All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco
• PR title is a worthy inclusion in the CHANGELOG

[Vasu1105]

LGTM

[PrajaktaPurohit]

LGTM!