Update INSTALL wrt missing dependencies
Update INSTALL wrt missing dependencies

This patch updates the quick installation guide with
with respect to some missing dependencies. Besides,
it rephrases some instructions to make them more clear.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Petr Pudlak <9d19625f8ee825a2c398462ed335cbcf98f79c32@google.com>

yiannist/ganeti,yiannist/ganeti,yiannist/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,leshchevds/ganeti,ganeti/ganeti,mbakke/ganeti,ganeti/ganeti,grnet/snf-ganeti,apyrgio/ganeti,grnet/snf-ganeti,dimara/ganeti,apyrgio/ganeti,andir/ganeti,andir/ganeti,bitemyapp/ganeti,leshchevds/ganeti,leshchevds/ganeti,onponomarev/ganeti,mbakke/ganeti,bitemyapp/ganeti,andir/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,mbakke/ganeti