lib.sh: Fix locals override in nested invocations

lib.sh: Fix locals override in nested invocations

'wait_for' bash function is nested in another 'wait_for' call in some
places, which leads to inner calls interfering with outer calls by
overriding the locally scoped variables, including the 'attempt'
internal counter. In some cases, the outer 'wait_for' would exit
after a single attempt.

Fix that by running all contents of `wait_for` inside a subshell,
which inherits outer calls variables, but does not override them
when the inner call is finished.

Change-Id: I450eda3d023af2380c61ee930071fbfc393a5645
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>