[KNIFE-464] Support EC2 STS, i.e. AWS Federation tokens for authentication

[chuckg]

KNIFE-464: Add support for the AWS session token to knife ec2 commands.

Effectively, this is a revert of a revert due to contributor license issues. I've rebased and fixed the tests. My opscode contributor user is cgg.

Related issue:
#276

Original merge (which was subsequently reverted):
https://github.com/chef/knife-ec2/pull/211/files

There were 3 tests failures, but those same 3 failures currently exist on master:

rspec ./spec/unit/ec2_server_create_spec.rb:436 # Chef::Knife::Ec2ServerCreate when configuring the bootstrap process configures the bootstrap to use prerelease versions of chef if specified
rspec ./spec/unit/ec2_server_create_spec.rb:1009 # Chef::Knife::Ec2ServerCreate tcp_test_ssh should return false if we do not get an ssh header
rspec ./spec/unit/ec2_server_create_spec.rb:1016 # Chef::Knife::Ec2ServerCreate tcp_test_ssh should return false if the socket isn't ready

[siddheshwar-more]

@chuckg @adamedx

To verify this we created IAM user with MFA enable option and used virtual device.
Tried - knife azure server list --aws-session-token <MFA-Code> . We are getting this ""' from device (i.e smart-phone in our case).
The generated MFA code is working with AWS portal. But not working with knife CLI.

Could you please send us steps/commands that you tried to test this changes?

[adamedx]

@siddheshwar-more, I think you want to pass the resulting sessiontoken value that you get from this aws cli comman

aws sts get-federation-token --name mystuff

rather than the MFA code.

[siddheshwar-more]

@adamedx @chuckg
I've retrieved session token with help of below command and tested this changes. Its working fine.

aws sts get-federation-token --name mystuff

@NimishaS We can merge this changes!

[NimishaS]

Cherry-picked these changes in #332 and merged