devstack: Clean up some of the devstack code
devstack: Clean up some of the devstack code

Mostly inspired by some comments left in the review of commit
aa5fd8afdc31b95c1a5f91f8af5fa733c4afd121 (Allow to set default ifaces
in DevStack)

Sorted the list of enabled interfaces and the default interface
values.

Used a local variable to store the uppercase value, in order to make
the code more readable as people may not immediately know what
${var^^} does in bash.

Change-Id: Ic2b2ac0a3cea9fa39173c5e2f5a9eaa3f4d8d2e4

openstack/ironic,openstack/ironic