oeqa: Streamline oe-selftest startup time

oeqa: Streamline oe-selftest startup time

"bitbake -e" executions from get_bb_var calls are slow and slow down oe-selftest
startup. Rationalise the code to avoid them and minimise the number of "parsing"
locations we use by caching key variables and passing them around more.

This was particularly problematic with oe-selftest -j usage since it would
have multiple bitbake -e executions per process making parallel usage
particularly slow.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>