Use 'st2 login' instead of modifying '~/.st2/config'

[arm4b]

Current installer writes the username/password pair in ~/.st2/config.

st2-packages/scripts/st2bootstrap-deb.sh

Lines 322 to 324 in 480f099

	[credentials]
	username = ${USERNAME}
	password = ${PASSWORD}

The new way is to use st2 login --save-password which is more smart and really authenticates and saves credentials.

Bonus: Apply the same enhancement to ansible-st2.