Check NOVA_USERNAME instead of USERNAME in openrc.

Normally, NOVA_USERNAME should be set to demo, but in some cases
it was being set to the running user ("stack") because it was
checking USERNAME as a possible default value.

This aligns with the pattern currently in master (for OS_USERNAME)
in that it uses either the value of NOVA_USERNAME or "demo".

This problem was identified by John Tran.

Change-Id: I82c005bbab9640642846793d25d36b776c8c897e
Use apt git package instead of git-core.

The git-core package is deprecated in favor of the git package. Use the
git package instead.

Change-Id: Ib136e34c7a0d4f87b02e32996420b3f507ca0bf4