Added token login to fly

[Chumper]

@vito
@jtarchie
@chendrix

Please review & merge.
This is linked to vmware-archive/atc#184 which needs to be merged first for fly to support token login.

This PR enables the user to do the following:

./fly login -c http://localhost:8080 -t main -n openCloak --token c8294f0dd721dbba7cf39a9ea882eed1273ab8ca
logging in to team 'openCloak'

Yeah, who needs the web auth flow anyway? Token FTW!

target saved

or

 ./fly login -c http://localhost:8080 -t main -n openCloak
logging in to team 'openCloak'

navigate to the following URL in your browser:

    http://127.0.0.1:8080/auth/github?team_name=openCloak&fly_local_port=56424

or enter one of the following token types:
    - Personal access token (e.g. 1233456789)
    - Bearer token (e.g. 'Bearer 1234567890'

Token: a5ea7763eb91fb0863cc8335879be34b7f8691fe

target saved

This also works with multiple auth methods.
Please note the failing tests because the atc PR is not merged yet.

[jtarchie]

@Chumper, it looks like the tests failed on this build.

[Chumper]

Yes, that is because the PR on ATC side needs to be merged first. It introduces the missing tokenUrl.
As the ATC tests are passing here https://ci.concourse.ci/builds/45871 I expect this tests to pass too after merging as it also passed locally.

[jtarchie]

please see comments

[jtarchie]

@Chumper, this http.Post bit us because it does respect the --insecure, which does skip ssl validation. Are you able to pass this as an option?

[Chumper]

Updated the PR to take the insecure option into account