cephadm: make the pip-install-from-source approach for deps explicit

cephadm: make the pip-install-from-source approach for deps explicit

Previously, the code assumed that all dependencies can be cleanly
installed from source with native-code compilers disabled. This worked
ok for MarkupSafe because it falls back to pure-python if it can't
compile the C extension code. Unfortunately, not all packages that can
fall back to pure python do so cleanly in the build. As a first step
to resolving that, make the current behavior an explicitly selected mode
and permit installing from wheels in the future.

Signed-off-by: John Mulligan <jmulligan@redhat.com>