Robottelo: Implement stubs from cli.test_myaccount.py

[renzon]

[renzon]

While checking into this issue I found that https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/cli/test_myaccount.py is very similar to https://github.com/SatelliteQE/robottelo/blob/master/tests/foreman/cli/test_user.py. Should I merge tests?

[elyezer]

I think merging them seems reasonable

[sghai]

I think My Account is more relevant to webUI, to manage user's self account from My Account menu. In CLI, we don't have any separate sub-command for my account so we should merge this.

[renzon]

@oshtaier pointed out on #3847 how my_account is different from user:

hammer -u ADMIN_USER -p ADMIN_PASSWORD update user --name MY_ACCOUNT --newname MY_ACCOUNT2

and

hammer -u MY_ACCOUNT -p MY_ACCOUNT_PASSWORD update user --name MY_ACCOUNT --newname MY_ACCOUNT2