PTS: Read SMP pairing options from bt_stack.conf

Use Case:
A number of cenrtification test cases require the host
to change the pairing options including auth request,
I/O capabilities, key distribution mask, key length
and Out of band capability.

Test Case: TP/PKE/BV-02-C

Failure:
Host currently only supports fixed SMP options and only
a few test cases involving pairing, are executed due
to that limitation.

Fix:
Added changes to read the SMP options from the bt_stack.conf
file at run time.
The pairing options are entered as a comma separated list eg:
SMPOptions=0xD,0x4,0xf,0xf,0x10
where the included hex values are in the following order:
\#auth, io, ikey, rkey, ksize
If all 5 options are not present, the host falls back to the
default values

Bug: 27852645
Change-Id: I693b05966695cf4660c5da10249c5f7c0663e53a