EL7 to use python3

[amanda11]

Update the EL7 packages so that they use python3 not python2.

Part of StackStorm/discussions#54

[amanda11]

Comments:

• Tested with docker images built by https://github.com/StackStorm/st2packaging-dockerfiles/pull/92, and then manual install on EL7.
• In EL8 we build cryptography. I haven't built cryptography for EL7, and it appears to use the same version of cryptography. Tested on a deployed EL7 with new packages that st2 key set/get with encrypted secrets worked.
• Also for EL8 in the st2.spec there are mention of need to get access to python packages available within BUILDROOT when outside venv, and it especially has BuildRequires in. I didn't need these steps to build the python3 EL7 ST2 package, but wanted to verify if those directives were used for anything other than building the packge.

[amanda11]

Tested 3.3 EL7 -> 3.4dev EL7 upgrade using new package. Manuallly install of libselinux-python3 before upgrade. Upgrade successful and self-check passed.

$ st2 --version
st2 3.4dev (632a07e), on Python 3.6.8