Correct VPNaaS limit checks in extension

Correct VPNaaS limit checks in extension

For the following VPNaaS attributes, ensure that these validation
tests are performed in the extension:

    keepalive >= 60 (seconds)
    mtu >= 0 (octets)
    DPD interval > 0 (seconds)
    DPD timeout > 0 (seconds)

Currently, the units for keepalive can only be seconds. If this
changes in the future (e.g. to allow kilobytes units), then the
test for the value will need to be changed.

To correctly test the MTU limits for an IPSec connection, the
protocol must be taken into consideration, which is defined by
the vpnservice object. Because of this dependency, we cannot
validate this in the extension, and will instead, just make
sure the value is positive.

Likewise, the attribute validators cannot ensure that the
DPD timeout is greater than the interval (again, because the
validators can only check the individual attributes one at
a time).

The range validator was modified to allow single ended ranges
for some of these attributes. The range also ensures the value
is an integer.

Companion changes are being made in the CLI code for these limit
changes.

Update: Fixed typo causing tox failure.

bug 1216117

Change-Id: I85d6175d81fbe7d27231aed48fb4691351e0db4c