rework IBMQ account adminstration

[ajavadia]

Summary

PR #1000!

Fixes #980
Fixes #1001

The IBMQ account adminstration has been changed a bit. Here is how it works now.

enable_account(token, url) # add to current session
disable_accounts(**kwargs)  #remove from current session
save_account(token, url) #save to disk
delete_accounts(**kwargs) #delete from disk
load_accounts(**kwargs) #load into session
active_accounts() # list all accounts active in this session
stored_accounts() # list all accounts saved to disk

Example usage:

from qiskit import *
import Qconfig
import QconfigCore

# start fresh. delete any accounts on disk
IBMQ.delete_accounts()
# verify that there are no accounts stored now
IBMQ.stored_accounts()

# enable an account (useful for one-off use, or if you don't want to save to disk)
IBMQ.enable_account(Qconfig.APItoken, Qconfig.config['url'])
# you see that specific account enabled for use
IBMQ.active_accounts()

# and its backends are available
IBMQ.backends()

# disable that account (so we go back to no accounts active)
IBMQ.disable_accounts(token=Qconfig.APItoken, url=Qconfig.config['url'])

# save two accounts: a public and a premium
import Qconfig
import QconfigCore
IBMQ.save_account(Qconfig.APItoken, Qconfig.config['url'])
IBMQ.save_account(QconfigCore.APItoken, QconfigCore.config['url'])

# now they should show up as present on disk
IBMQ.stored_accounts()

# but no account active in current session yet
IBMQ.active_accounts()

# so IBMQ can't see any backends yet
IBMQ.backends()

# now load up every account stored to disk
IBMQ.load_accounts()

# backends from two different accounts available for use
IBMQ.backends()

# now if you want to work with backends of a single account,
# you can do so via backend filtering
IBMQ.backends(name='ibmq_qasm_simulator', group=None)

# but you can also just disable account in the current session
IBMQ.disable_accounts(project='qiskit-core')

# so now only one account is active
IBMQ.active_accounts()

# and only that account's backends are available
IBMQ.backends()

# or from the start just load up that account you're interested in
IBMQ.disable_accounts()
IBMQ.load_accounts(project='qiskit-core')
IBMQ.backends()

[ajavadia]

@diego-plan9 please take a look at this to make sure I didn't mess up anything with your account management stuff.

[ajavadia]

Please hold off on merging until I push a small fix.
Some tests fail locally.