CBQE-929: change sync/async erlang threads via installer.py

now we have possibility to change only a number of threads for async type
these parameters can be used to set type and number of erlang threads:
 sync_threads=True       Sync or acync threads(+S or +A)
 erlang_threads=         Number of erlang threads (default=16:16 for +S type)

example:
 -p erlang_threads=128:128 will set +S 128:128
 -p erlang_threads=32 will set +A 32
 -p sync_threads=True,erlang_threads=16:16 will set +S 16:16

Change-Id: I7500b517b48de6be2dc0b2e4133a55b8ad11723f
Reviewed-on: http://review.couchbase.org/23541
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>