app: check DEFAULT_LVM_POOL variable for default pool
app: check DEFAULT_LVM_POOL variable for default pool

Allow to hint default_pool what is default storage pool - especially
useful for tests.

marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin