Changes

ss7 vty: Set default route priority to 5

Use a priority != 0 (best prio) as default priority. This allows adding
manual routes with higher prio (lower value) more easily.
Value 5 is chosen to resemble other systems users may be already used to.

Changing the default priority value in routes now shouldn't be much of a
problem, since the only way to set up a specific priority (VTY) was
broken until now and would return an error, so we know for sure nobody
has been setting priorities other than the default value.
Hence, with this change even older implementations reading a backward
compatible file would still work the same by using all-prio-0 instead of
all-prio-5.

The "update route" VTY cmd gets a new "default" param value for the
"priority" param, hence getting some syntatic sugar which also helps
users understand what's the default value through the CMD help string.

Change-Id: I4e283cc07b8e9ea2124ec9da18cbce77c3d866ee