armband: Enable deploy-cache

armband: Enable deploy-cache

This enables the mechanism added in [1].

Armband just added support for caching deploy artifacts, like
bootstraps and target images, which take a lot of time at each deploy
to be built, considering it requires a cross-debootstrap via
qemu-user-static and binfmt.

For easier setup, the deploy cache depends on the local ISO storage
mechanism (/iso_mount/opnfv_ci), and reuses the same storage pool.

To enable deploy caching, just create the deploy-cache directory at:
/iso_mount/opnfv_ci/<branch>/deploy-cache

JIRA: ARMBAND-172

[1] https://gerrit.opnfv.org/gerrit/#/c/25191

Change-Id: I161205a87f13cd78e79b96bfc37a5f68aba093cc
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>